Version .5
Let me know if something is wrong, you've found a better method or tweak(especially for the immune enemies), or this bug is patched out. Thanks!
Version .55
Added that some enemies can only be killed with one of the methods listed.

I found the basics of this method on YouTubers "jashinsamael" video
"[GE2RB] God Eater 2 Rage Bugs #1 - +99 Deadly Venom (Literally) - Steam ver. only"

It shows a bug in how GE2RB calculates venom damage on the Steam version. With extreme enemies like you find in Lvl 99 missions, the Venom debuff will lower them to one hp as soon as it fully activates. I've built and expanded upon this method. It won't work on every enemy however so some missions can't be beaten by this alone. For enemies it does work on, they will die in seconds. This includes bosses like Crimson Orochi, Last Vestige, etc. This guide will describe the Melee Venom Method and the Ranged Venom Method.

This list in not fully tested.
Note-Some enemies can only be killed with either the Melee OR Ranged method but not both.
Tezcatlipoca
Ouroboros
Fallen Ouroboros
Venus(On Hard Level 15:Petrov Defence)
Chi-You(On Hard Level 15: Intake Manifold)
Abaddon

Nyx Alpha(with melee vesion)

To beat these enemies is outside the scope of this guide but I recommend using Divine weapons. If you find a good method for these or a correction, let me know.

The key to this method is giving the enemy the Venom debuff and being on a very high perilous mission level. Once it fully activates on an enemy at Lvl 99(takes about .5 seconds) their HP drops to one and another hit will kill them(with a few exceptions). You'll know it activated when you see the purple mist or hear "..venoms coursing through its blood". For some immune enemies it will still activate but not kill them.
NOTE: Poison for bullets and Toxic for Personal Abilities also give Venom.

The four methods I know of are to give the Venom debuff are
1. Venom Trap
2. NPC abilities
3. Poison Bullets
3. With a melee weapon and the Venom skill(try for at least Venom skill level 8)

You can only carry one venom trap and they are annoying to make so we'll skip them. The NPCs personal abilities "Venom Hits" and "Toxic Bullet(and variants) can also work but have a low activation rate. So we will mainly focus on melee weapons and poison bullets.

Poison bullets become available before you unlock Perilous Challenge missions, so you don't have to do anything special.

Weapons can only get the "Venom" ability by already having it or through upgrade(though this is just a higher version usually). It can't be added by abandoned god arcs. Also even though "Venom" skill can be found on a gun, it still only will gives the melee version. You will need to use poison bullets with a gun. Also according to the wiki though the level effects the length of the debuff, which doesn't mater to us, my testing seems to show higher level Venom abilities will debuff the enemy much faster.

Once you've unlocked Perilous Challenge you can start.

Melee Venom Method at its most basic can be effectively achieved with weapons as low as short blade "Traviata" and sniper "Manon Lescaut" for a total of Venom 8. This can be vastly improved with Link Aids, skills(such as Hidden Attack Lvl 10), and NPCs.
Note-Some enemies can be immune to just one of the Melee or the Ranged toxic methods. A select few are immune to both.
https://youtu.be/rLLh3GiDlZk

Once you have the Venom skilled weapon ready and have the highest Venom Lvl you can get(see "Venom Weapons" section), start a level 99 mission and MELEE hit the enemy a few times to activate the debuff. When you see it activate(purple mist) or hear "..venoms coursing through its blood" one more hit(melee or gun) will kill it unless it's immune.

Starting out I recommend
-Short Blade for its speed
-Sniper rifle for its "Stealth Field"
-Tower shield for its protection
-Skill Hidden Attack Lvl 10 if available.

Also if you're not soloing ALWAYS take Romeo for his Heart blood power so enemies have a hard time sensing you.

Ranged Venom Method is just a gun with a poison bullet at it's most basic but works MUCH better with Hidden Attack Lvl 10 which increases the debuff chance on out of combat enemies.
Note-Some enemies can be immune to just one of the Melee or the Ranged toxic methods. A select few are immune to both.
https://youtu.be/6odjuSG5tZA

Once you have a poison bullet and Hidden Attack skill Lvl 10, start a level 99 mission and shoot the enemy. You should quickly see it activate(purple mist) or hear "..venom's coursing through its blood" one more hit will kill it unless it's immune.

Starting out I'd recommend trying using :
Sniper - for its range and "Stealth Field" ability.
Hidden attack skill Lvl 10, can be on weapon/shield or Upgrade Parts.

If available these can help if there is more than one enemy in the mission or you are using something other than the sniper rifle.
Use Romeo for his Heart blood power(so the Aragami don't see you)
Link Aid : Blind 0-2, Blind 2-5, and some speed or full burst aids.

Here is a safe setup for soloing quick missions. For this you will use Stealth Field to get up to the enemy unseen and melee them.

-Short blade "Warlord Knife Umbra" for Venom 10(or other fast Venom 10 weapon)
-Any sniper rifle for "Stealth Field"
-Hidden Attack lvl 10 skill to increase the debuff chance.
Optional
-Link Support: "Blind 30% 0-2", "Bodyguard 0-2", "Full Burst 0-2", and "Speed 20% 0-2". Could use another "Blind 0-2" in place of Speed.
-Skills like Athlete and Stamina to get you to the enemy fast.
-Skills like Firm Stand lvl 10in case you get hit.
-Skill Burst Duration to keep the burst going
-Skills like Muted lvl 10 can be used to but Romeo is much better.
-I like to use a Tower Shield and Auto Guard. Even better with Prepared, Guard Speed and Guard Radius

https://youtu.be/HMx9g7u08zI

You can also skip using the "Stealth Field" and bring whatever gun to solo rush them. Good if you want to bring an assault or blast gun build for the immune enemies.

https://youtu.be/KCdOqy6Q6Ho

The basic setup is the same as solo but
ALWAYS take Romeo for his Heart blood power to make the enemies nearly unable to track you and increase your chance of getting Hidden Attack to work.
Have other NPCs as healers, burst providers, and give them some Venom personal abilities if you want. Also Nana isn't great to bring.

The safe way would be to use basically the same as the Ranged with Melee backup build under the Longer Mission Builds Combo Methods section. This is due the fact that the Ranged Venom Method will aggro the enemies and if the first few hits don't kill them, melee is the fastest way to eliminate them at that point.

See Melee with Ranged Backup under Longer Mission Builds Combo Methods

The basic setup is the same as solo but
ALWAYS take Romeo for his Heart blood power to make the enemies nearly unable to track you and increase your chance of getting Hidden Attack to work.
Have other NPCs as healers, burst providers, and give them some Venom personal abilities if you want. Also Nana isn't great to bring.

For soloing longer missions sort of safely one way is using primarily Ranged with Melee as back up.
With this you try to hit the enemies at range, use Stealth Field, and take advantage of Hidden Attack. If the debuff doesn't work in the first few shots the Hidden Attack benefit failed and you're better off using melee to quickly kill the aggroed enemy. Things like Link Aid Body Armor and skill Firm Stand will help in those situations.

Use
-Short blade "Warlord Knife Umbra" for Venom 10(or other fast Venom 10 weapon).
-Any sniper rifle for "Stealth Field".
-Sniper poison bullet(I use a Small poison with Multi-Hit) .
-Hidden Attack lvl 10 skill to increase the debuff chance.
Highly recommended
-Link Aid - (Blind 0-2, Blind 2-5, Full Burst 0, 2, and Body Armor 0-2).
-Skills to keep Oracle up(Oracle, Frugal, Trigger Happy, B Oracle Auto-Recovery)
-Skills to keep Stamina up(Stamina, Athlete).
-Skill Burst Duration to keep burst up.
-Skill Muted lvl 10 to stay quiet.
-Skill in case you get hit or need to go melee(Auto Guard lvl 10, Prepared lvl 10, Firm Stand lvl 10, Guard Speed lvl 10).
-Tower shield for best protection.
-Healing Items in case you need them.

Sniper Poison Bullet
on button press - poison - sniper: sniper/normal - Multi-Hit - 0,0,0


https://youtu.be/a2aUFNfOK5E
NOTE: If any of theses enemies didn't get debuffed in the first three or four shots I would have gone in for the melee(except Nyx Alpha, can't melee them).

This is much more risky solo as you don't have Romeo's Heart numbing the enemy's senses or NPC healers. The assault gun can be used for immune enemies or with a poison bullet.

The key here is a fast weapon(Venom skill 8+) and protection.
Use
-Short blade "Warlord Knife Umbra" for Venom 10(or other fast Venom 10 weapon).
-I brought Ipalnemohuani for good Divine attacks on immune enemies.
-Strong Tower Shield
-Link Aid - (Blind 0-2, Body Armor 0-2, Full Burst 0, 2, and Speed 0-2). Or similar.
-Firm Stand lvl 10 to stay alive.
-Skill Muted lvl 10 to stay quiet.
-Skill Burst Duration to keep burst up.
Highly Recommended
-Skills to keep Oracle up(Oracle, Frugal, Trigger Happy, B Oracle Auto-Recovery) in case you go ranged.
-Skills to keep Stamina up(Stamina, Athlete).
-Skill in case you get hit or need to go melee(Auto Guard lvl 10, Prepared lvl 10, Guard Speed lvl 10).
-Skill Hidden Attack lvl 10 isn't going to hurt if you feel like using some ranged or are sneaky.
-A poison bullet just in case.
-Healing Items in case you need them.

Btw the bullets I use with assault for immune or non high level enemies are. They are not my creations. Please see here for much more on them
https://cs2bus.com/sharedfiles/filedetails/?id=759919427

Charger(fills oracle)
on button press - Medium -non elemental - rapid: rapid/normal - Three-Shot Replica. - 0,0,0


Elemental Inf Homing (one build for each of the four elements)
on button press - medium - non elemental - rapid : rapid/normal - homing - 0,0,0
--With 1 - large - elemental - shot: straight/point-blank - no chip - 0,0,0


Elemental Inf Straight (one build for each of the four elements)
on button press - medium - non elemental - rapid: rapid/normal - Three-Shot Replica. - 0,0,0
on button press - SS - elemental - shot: straight/short - Enhanced Accel. - 0,0,0
--when 2 hits foe - large - elemental - shot: straight/point-blank - no chips - 0,0,0


Poison Bullet
on button press - poison - medium - rapid: rapid/normal - homing - 0,0,0


Put Charger on x and Elemental Inf Homing or Elemental Inf Straight on y and just mash them. With trigger happy, frugal, and good oracle and stamina Charger will replenish oracle as the damage bullet does its thing.

https://youtu.be/5WhES0XCdJc

These are the same as solo but bring Romeo for Heart blood power and have Hidden Attack lvl 10. Use NPCs for healers, burst providers, and give them some Venom personal abilities if you want. Also Nana isn't ideal to bring.

https://youtu.be/1-tXL6B-vfk

https://youtu.be/4fyWFONxdeM

At 99 your not usually using NPCs to kill(except with Venom) but for their Blood Power, keeping you in burst, and healing.
NOTE: I believe you get more burst from AI if you get rid of the Link Bullets they give you. You can use their Mutual Upgrade with your own Mutual Burst skill to keep your Burst up.

Romeo is the by FAR the best NPC to bring once he gets his Heart blood power. It makes it so enemies have a very hard time sensing you. The result is you stay alive and can Hidden Attack the enemies much more efficiently.

Nana is...not so good. Drawing enemies at 99 is an option but probably not the best one.

Julius' Control(grants burst shift) and Livie's Kindness(gives Raise) are ok but not great.

Ciel Intuit and Gil's Inspire powers are not so useful.

I recommend focusing on these things for the personal abilities.
-Staying Alive
-Healing
-Giving Burst
-Rewards
-Giving Venom debuff(can be skipped)

Staying Alive
The usual things go here
-HP Up
-Damage Cut
-etc.

Healing
-Various healing bullets
-Any character specific PAs that increase healing ex. Kanon's(Responsibility) and Ciel's(Self Control)

Giving Burst
-Mutual Upgrade III and higher to increase burst to three and increase frequency.

Rewards
Whatever end mission rewards your going for.
Non-Operators
-God of Rare Drops
-Rare Items for All
-Share the Wealth
Operators
The usual, whatever you're farming
-Spports Skill + 1
-Support Redraw
-Rewards Up
-Ignore Bonds
-etc.

Giving Venom debuff
-Venom Hits
-Any Toxic Bullet

This list may not be complete but it should be plenty for use.
NOTE : When a weapon has for example Venom 4 and you get Venom 6 both can be slotted(separate slots) to get Venom 10

Datura Rank 2 (Venom 2, Venom Resistance 5)
--Albertus Rank 4(Venom 4)
----Cantarella Rank 7(Venom 6,Silent Killing 10)
------Cantarella EX Rank 11
--------Cantarella W Rank 15(Venom 10)

Traviata Rank 4(Venom 4)
--Falstaff Rank 9(Venom 6, Bacillus 10)
----Falstaff EX Rank 12
------...

Warlord Knife: Woe Rank 11(Venom 6)
--Warlord Knife: Pain Rank 12(Guard Radius 12)
----Warlord Knife: Umbra Rank 15(Venom 10, Destruction Blow 10)

Falstaff EX Rank 12(Venom 6, Bacillus 10)
--Falstaff W Rank 15(Venom 10, B Combo Master 10)

Warlord Blade: Woe Rank 11(Venom 6)
--Warlord Blade: Pain Rank 12(Guard Radius 10)
----Warlord Blade: Umbra Rank 15(Venom 10, Destruction Blow 10)

Warlord Edge: Woe Rank 11(Venom 6)
--Warlord Edge: Pain Rank 12(Guard Radius 10)
----Warlord Edge: Umbra Rank 15(Venom 10, Destruction Blow 10)

Pitohui Mallet Rank 2(Venom 2)
--Irukandji Mallet Rank 3(Venom 2, Venom Resistance 5)
----...
--Cronus Mallet Rank 4(Venom 2, Venom Resistance 5)
----...
--Botulinum Mallet Rank 5(Venom 2, Venom Resistance 5)
----...
--Paraponera Mallet Rank 7(Venom 4, Poison Resistance 10)
----Slow Loris Mallet Rank 11(Venom 6)
------Blue Ringed Mallet Rank 15(Venom 10, Debuff Stacking 10)

Irukandji Mallet Rank 3(Venom 2, Venom Resistance 5)
--Gaboon Mallet Rank 7(Venom 6)
----Gaboon Chain Mallet Rank 11(Venom 5,Venom Resistance 10, Air 10)
------Gabonica Mallet Rank 15(Venom 10)

Cronus Mallet Rank 4(Venom 2, Venom Resistance 5)
--Belcher Mallet Rank 7(Veom 4)
----Boomslang Mallet Rank 11(Venom 6, Venom Resistance 10, Air 10)
------Ophiophagus Mallet Rank 15(Venom 10)

Botulinum Mallet Rank 5(Venom 2, Venom Resistance 5)
--Anisatum Mallet Rank 8(Venom 4)
----Yew Mallet Rank 11(Venom 6, Venom Resistance 10, Air 10)
------Pieris Mallet Rank 15(Venom 10)

Warlord Maul: Woe Rank 11(Venom 6)
--Warlord Maul: Pain Rank 12(Guard Radius 10)
----Warlord Maul: Umbra Rank 15(Venom 10, Destruction Blow 10)

Freak Rose Rank 2(Venom 4)
--Freak Opal Rank 5
----Freak Beryl Rank 8(Venom 4, Air 10)
------...
--Freak Ruby Rank 7(Swift Strike 10)
----Freak Blood Rank 11(Venom 6)
------Freak Rubellite Rank 13(Debuff Stacking 10)
--------Freak Garnet Rank 15(Venom 10)

Freak Beryl Rank 8(Venom 4, Air 10)
--Freak Jasper Rank 11(Venom 6)
----Freak Chalcedony Rank 14(Hidden Attack 10)
------Freak Amethyst Rank 15(Venom 10)

Warlord Spear: Woe Rank 11(Venom 6)
--Warlord Spear: Pain Rank 12(Guard Radius 10)
----Warlord Spear: Umbra Rank 15(Venom 10, Destruction Blow 10)

Curare Rank 7(Venom 2)
--Curare II Rank 9(Venom 4, Jamming Resistance 5)
----Curare EX Rank 14(Large Shield 10)
------Curare W Rank 15(Venom 6)

Harvest Rank 8(Venom 4)
--Harvest II Rank 10(Venom 6, Silent Killing 10)
----Harvest S Rank 13
------Good Harvest Rank 15(Venom 10, Debuff Stacking 10)

Warlord Spear: Woe Rank 11(Venom 6)
--Warlord Spear: Pain Rank 12(Guard Radius 10)
----Warlord Spear: Umbra Rank 15(Venom 10, Destruction Blow 10)

Manon Lescaut Rank 5(Venom 4, Mid-Air Jump 10)
--Grand Guignol Rank 9(Venom 6, Melee Spcial ATK Stam. 10)
----Orlando Rank 12
------Turandot Rank 15(B Debuff Stacking 10)

NONE

NONE

NONE