This guide is aimed at providing some info on the combat system and related parts in game.
It is still far from complete and possibly has a few mistakes.
I will try to finish it over time.

In combat you'll decide which weapon Helen should use to fight at the start and after it and so on.
Weapons have 3 stats:

• Effect: The power of the weapon, determines how much damage you deal or with healing spells how much they heal you by.
  
• Defense: How much less damage each of your opponent's attacks will deal whilst you have that weapon readied.
  
• Wait: How long before the weapon does it's effect (if it has one) and then allows you to choose something else to do (if the opponent has not been defeated yet)

Over the course of the game you'll come across a wide variety of weapons to use these will be explained in detail in the next section.

Lastly some weapons have special effects:

• Piercing: The attack ignores all enemy defense.
  
• Heals: The item does not deal damage but heals you instead.
  
• Stun X: The attack causes your opponent to wait the amount specified in X
  
• Absorbs: The attack heals you for how much damage it deals.
  
• Sealing: The attack seals the opponents currently readied attack preventing it from being used.

*NB: This section is not complete.

The rules of combat apply to both you and your opponent.
Pay attention to how much power the enemies attack is, how much defense they have and how long before they hit.
Piercing attacks are great when the opponent is currently guarding.
Use quick attacks in between your opponents attacks to get off extra damage.
Use defensive equipment when your opponent is about to use a powerful attack but only when you when you have no time left.

A couple things to note.
The opponents starting attack is unknown. It's best to use either a very quick move or a defensive move as your starting move.
If both yours and the opponent's wait end on 0 you get to hit the enemy and choose your next move before they attack. This can be highly useful with the Longbow at the start as many enemies have 10 wait and it has 5 allowing 2 free shots before the opponent can retaliate.

Here I will talk about the different types of equipment.

The first type you get access to is the Bow type equipment:

• Low Effect
  
• Low Defense
  
• Short Wait

Then is the Shield type equipment:

• No Effect (Barring a late game exception)
  
• High Defense
  
• Mid Wait

Then is the Sword type equipment:

• Mid Effect
  
• Mid Defense
  
• Mid Wait

Then is the Spell type equipment:

• High Effect (For damaging spells others tend to have other useful effects)
  
• No Defense
  
• Long Wait (Most when upgraded shorten their wait time)

Lastly is the Passive type equipment:
The passive equipment are different in that you do not use them like the other equipment but they always apply their benefit to all your other equipment.
There are passives that increase (or decrease for wait) each of the stats.

HP: 18
XP: 2

Battle Master:
"Slimes are causing you trouble, are they?
They move really slowly,
so just use your shield to block their attacks."

HP: 24
XP: 3

Battle Master:
"Goblins getting you down, huh?
They often work themselves into a frenzy.
Defend when that happens, and then go in for the kill!"

HP: 15
XP: 3

Battle Master:
"Watch out, Yhere are Orcs around here.
They have high attack and defense,
But their constitution isn't very good.
They move pretty slowly, too. Try using a powerful move on them!"

*'Yhere' is probably typo and should be 'there'

HP: ~20
XP: 3

Battle Master:
"Did you notice the Skeletons all seem to fight using nice long swords?
Did you notice?
It's a good idea to prepare for all types of enemies, but sometimes you just have to believe in yourself!
If you know what you're doing you could use your EXP to forge new weapons!
... probably"

HP: 40
XP: 5

Battle Master:
"You fought a Mummy?
They're pretty strong, and they have a skill to change the tempo of battle.
They're just all around mean guys!
Fortunately, their attacks are really slow.
It'll be tough, but you can do it!"

HP: ~74
XP: 20

Battle Master:
"Golems are tough!
There's no way you can bring them down unless you give it your all!
I'm sure you can figure out a way to beat them."

HP: ~41
XP: 4

Battle Master:
"Whatever you do, don't drag out a battle with a Zombie!
Their HP is high, but their defense is low!
If you know how, some rapid fire arrows should put them down."

HP: 81
XP: 14

HP: ~28
XP: 4

Battle Master:
"You saw a Ghost?!
They chant THREAT, and they can use magic, too.
If you can, use a light weapon against them."

HP: ~32
XP: 6

Battle Master:
"Harpies are really smart.
If you give them any opening, they'll pepper you with attacks!
They're pretty good at defending, as well.
But I think if you have some way to attack even quicker, you'll be OK!"

HP: ~100
XP: 30

Battle Master:
"A Griffin means big trouble!
They're smart, and they're fast.
If you use a slow attack, you're going to be in trouble.
But there's no such thing as an invincible enemy!
It takes time for them to prepare their next attack, after all..."

HP: ~2
XP: 10
*NB - The Disappear skill does not display any effect. The skills effect prevents you from hurting the opponent whilst it is in use. To be more specific it cancels your move and even Piercing attacks won't work.

Do not be fooled by the low hp or it's comical apperance.
This foe knows how to use the skills it has to the fullest.

Strategy:
The only way to beat it is to bring the Quick Move, Quick Heal and preferrably a fast weapon.
Start with the Quick Move and then use Quick Heal to keep your hp up.
Continue that until your weapon gets to 1 wait.
Use the weapon the moment the opponent is waiting on one of its attacks.

HP: ~257 (I strongly believe it's around 250)
XP: 50

Strategy:
Spike Guard is a good choice, bring along Heal, Quick Move, Speed Shoes, and whatever else you'd like.
He seemingly always starts off with Yasha's Blizzard so feel free to use some long wait item, I use the Quick Move so as to not be as severely affected by the Waitup of the Blizzard.
When he uses Freezing Yasha use Spike Guard to nullify his damage (also preventing the Max HP Drop effect) and deal some nice retaliation damage.
Use Heal when low on HP.
Continue until he is defeated.

HP: ~100
XP: 30

HP: ~81
XP: 24
*NB - Super Mimic is a unique skill that copies whatever skill you are doing, however with a few differences. The copied skill has the Effect +5, the Defense +5 and the Wait -2 of your skill's basic stats.

HP: ~100
XP: 30

HP: ~126
XP: 4

HP: ~6
XP: 1

HP: ~160
XP: 31
*NB - Exciting Slots! increases in Effect and Defense by 11 for every 1 wait that passes, it starts at 55 and goes up to 99 and goes to 0 and so on. Also the increase will stop at the current number when hit until the next time it starts using Exciting Slots!

HP: 450
XP: 35
*NB - Demonblade takes half of its current hp as damage each turn.
*NB - Demonblade also seems to follow a preset pattern of Slash, Slash, Delusion and Demonblade skills, it will repeat this pattern for as long as the battle lasts. (This shouldn't last more than once though)

HP: ~146
XP: 40
*NB - The 'Lost HP = Bonus Effect' ability takes half of the damage you deal and adds that to the effect of the attack. Also this added effect will stay permanently throughout the battle. Ideally you want to finish the battle as soon as possible to minimize the damage he deals to you.

HP: ~146
XP: 10
*NB - This is another tricky fight as he is rather adept at countering your moves also his attacks are pretty fast considering their power.

HP: ~300
XP: 200
*NB - It's possible to trick Ahriman into doing things 'your' way so to speak.

I wasn't paying a lot of attention to the HP counter or the Wait times of the various moves.