I'm on the idea that if you use a weapon, it's a waste not to put points in its related skills.
To avoid this, the thing is simple, don't use that weapon.

The engineer has skills that help any melee 2-handed weapon beside the staff (apparently not true, but it's a bug[forums.runicgames.com]), and any melee 1-handed weapon. Respectively Heavy Lifting and Supercharge.
Supercharge can also help 2-handed weapons but I want to focus the attention about this skill in another sitution. Keep in mind it's a great skill for a Dual Wielding build.

For ranged weapons it's simple because only cannons benefit from ranged skills. Usually don't take in consideration bows, crossbows or shotguns.

Picking a weapon means 15-30 points already placed but what about pistols?
They are ranged true, but can be 1-handed and are fast.
These are important things, because can trigger effects fast and because can be equipped with shileds.
Shields can have a proper build, but are usually used regardless point investment because of the survival task.

Pistols don't make me to invest points in any melee or ranged skill, all free points can help to build a summoner or a shield engineer, the mix?

The Robomancer
.

Robomancer, 16/60/55[ralrom.github.io]

1 point left which can be placed anywhere.

The build is strong and well balanced when the minions damage is boosted enough, the main problem is the early game in elite mode, when the damage lacks.
Before unlocking Spider Mines use a good shotgun to keep creature out of melee.
At level 14 things'll be easier.

• Ember Reach, 15/15: it boosts minions and Shield Bash damage. Core.
  This skill is good, really good. You can do a lot of tricks. Ember + Shield Bash, Spider Mines +Ember, interrupt and pull ranged or mages.
  This skill is so important because has a damage physical buff and the build is based on physical damage: minions, shield bash.
  
• Storm Burst, 1/15: 1 point, it's for reposition purpose.

• Healing Bot: 15/15: core for any build, keep max out.
  
• Spider Mines, 15/15: first damage skill.
  
• Gun Bot, 15/15: another bot for the robomancer.
  I find this bot better than Sledgebot, once placed it melts everything in no time.
  
• Sledgebot. 15/15: another bot with you.

• Shield Bash, 15/15: this is a physical build, must have, stun, another main source of damage and charge generation.
  
• Forcefield, 15/15: must have.
  
• Tremor, 15/15: huge range, huge crowd-control, huge defense debuff.
  
• Immobilization Copter, 10/15: it's mainly used to snare, the damage is a side effect (because it's electrical) that's why I think 10 points are enough; that chance to interrupt isn't worth 15 points.

Suggested place for the 1 point left:

• Seismic Slam, 1/15: can help during the early game.
  
• Coup de Grace, 1/15: can combo well with Shield Bash and Spider Mines, but isn't reliable on bosses.
  
• Charge Domination, 1/15: a random hit can refill your charge bar, but this build isn't focused on the charge use.
  
• Charge Reconstitution, 1/15: after using Forcefield, the health regen can be handy in any situations.

Being a summoner build you can put points into only one stat: Vitality.
Anyway it's not meant to be a regular summoner build because there's Shield Bash, pistol and physical damage.

A deciding factor is Spider Mines, this is one of the main skill and it consumes a lot of mana,
Not only, the other main skill, Shield Bash, scales with Focus.

It's pretty obvious that FOCUS is 1 of the main stat of the build.

Anyway I don't reccomend to put only points in FOCUS or in VITALITY.
I recommend to have a well balanced stat build.

Speaking about the first walkthrough from level 1:

• Put constantly 1-2 points in Vitality, because good gear needs vitality points to unlock.
  
• Spare points in Focus, Dexterity, Strength, just focusing only on one when you need to unlock a good gear.
  Focus helps you to handle the mana pool, remember to use mana regen sockets too.
  Strength and Dexterity make crits from Shield Bash scary.

The stats after defeating the Netherlord in elite mode:

STR: 96
DEX: 32
FOC: 103
VIT: 79

After the first walkthrough the goal is always the same:

• Boost any pet/minion damage with items;
  
• cap the strength to 1000 points (for the crit damage);
  
• set enough points in dexterity to have at leat 20% crit % and decent dodge %;
  
• put the minimun points needed in vitality to have 75% to block (don't forget shield bonus);
  
• boost focus or keep putting points into vitality.

The spells have the target to boost minions damage and create an army.

• Animal Handling[forums.runicgames.com],
  it is a sure pick, but it works with your pet and your summons, not with pet's summons.
  You need another Animal Handling learnt by your pet to boost his summons.
  
• Summon Skeleton,
  It's better you learn this spell instead of your pet cause the low cd, 8 seconds, and because it'll be boosted by Animal Handling. With that low cd you can have more skeletons at one time.
  
• Summon Archers or Summon Zombie,
  Archers have a long cd, but they are perfect for bosses or tough guys.
  Zombies have a good cd, you can always have a pack summoned and they are usefull cause of the poison.
  
• Concentration -> Blocking
  At the beginning Concentration helps a lot with the mana. When this problem is solved pick Blocking.

Because of the Animal Handling's behavior, I recommend to take summons for support purposes, like the imps.

• Silence,
  good range and has a big impact vs a pack of monsters.
  
• Summon Nether Imp,
  can cast silence.
  
• Summon Nether Imp.
  
• Heal All or Frost,
  Heal All can help you and your minions stay alive, but the pet also uses it when out of combat.
  Frost is a nice spell, the damage is good, 100% freeze makes this spell a great crowd-control.

I don't want to do a list of what gear use, googling already gives you a good list of the best pieces to use and then, it's always funny to find out the best set or gear for your tastes.
In the References section you can find two links for the gear.

One thing is sure: socketables[torchlight2armory.com] that improve minions' damage.
This is the only way you can make effective your minions, and has a big impact into the game.
This means that if you don't improve your minions you'll have troubles in elite mode.

For the weapon set I want to do a digression.
I recommend two set:

• sword/shield: classic Irow Sword with pet damage, switch into this when you aren't active in combat;
  
• pistol/shield: the native set of this build.

Pistol fits well in this build and has an important role.
Pistols can have crazy speed attacks, and I think this is the feature to focus on.
Try to find a very fast pistol and take advantage of the DOTs or the cast spell mechanic.
DOTs and spells' mechanic is explained in the next 2 sections but the main concept is: DOTs scale with focus and stack.
With a fast pistol the Robomancer becomes a good damage supporter for his army.

It's better using the DOTs instead of the spells (like Thunder from Yellow Basilisk Eye[torchlight2armory.com]) because can stack faster with 100% success.
Spells work better with weapon DPS skills or in other situations (a weapon affix with better %, dual-wielding weapon with high execution %, spammable weapon DPS skills).

DOTs can be applied by skills or by weapon's affixes, anyway their stacking behaviour is different, from this topic[forums.runicgames.com]:

• DoTs from skills never stack with themselves: you can cast Magma Spear as many times as you like, but the target isn't going to get more damage per second (however, the duration will be refreshed each time). Do note that the number in the icon that tells you how much damage the target is taking does change for some skills, as some DoTs have a damage range and a new application can roll a different number in that range. DoTs from skills usually scale with focus and the appropriate +% damage modifiers.
  
  
• DoTs from weapons (as a native affix, not as a socket - enchantments not tested at this time) DO stack with themselves [edit: apparently so do DoTs from enchantments - thanks lminith]. Two hits from Endgame will result in the target taking twice the damage over time, with the game keeping track of each duration separately (the icon will only show the longest time remaining). In addition, those DoTs scale with your character's Focus, but not his strength (even though technically they're physical damage - go figure). In other words, they can be a source of massive damage if you have a fast-attacking weapon or a fast-casting skill, and lots of Focus.
  
  
• DoTs from socketables stack with themselves too, both from multiple weapon hits or from having multiple socketables in one weapon. However, they don't scale with Focus (nor Strength), so their damage ends up rather piddly. Not worth the socket IMO. (edit: apparently fixed to scale with Focus in 1.22 - according to patch notes - but I haven't tested it)
  
  
• Also, as a miscellaneous bit of information: DoTs don't build charge, whether from a skill, weapon or socketable.
  
  
• Armor debuffs from weapons or socketables do stack with themselves too. More interestingly, the "% chance to flee" debuff (which I assume works like other "chance to inflict X" affixes) seems to stack too, though I have no idea how that works: if you hit the dummy with a "10% chance to flee" weapon, once every few hits it'll get a little icon that says "10% chance to flee". If it gets that effect applied again, the icon will be upgraded to 20%, then 30, etc. At face value it's like you have a chance to inflict a debuff that has a chance to cause the target to flee; my guess is that the game is just acting up a bit and the 20/30/40% chances just mean the "fleeing" debuff was applied while it was still active from a previous hit.

Affixes are the effects attached natively or with enchantment to a weapon.
Affixes can be +X to a stat, % to raise a minion, % to cast a special spell like Thunder or Glacial Spike or Meteor Strike.

All these effects can be triggered when auto-attacking but there are skills that apply the weapon affix too. Not all the skill can, but some based on weapon DPS, from this topic[forums.runicgames.com]:

• Skills that are NOT based on weapon DPS do not transmit any active effect (i.e. DoTs, % chance to cast spells, armor debuffs, etc.).
  
• Skills based on weapon DPS may transmit some, but it depends on the effect and on the skill.

Meanwhile these infos are useless for a summoner build, talking about Robomancer, they are usefull when planning builds around weapon DPS skill.

Keep in mind that spells like Glacial Spike and Meteor Strike scale with character level and are affected by Focus.

GAME MECHANICS, how works damage.[forums.runicgames.com]
GAME MECHANICS, triggering spells.[forums.runicgames.com]
GEAR, If you need more informations about the gear for engineer summoners.[www.urgametips.com]
SPELLS, generic discussion about the summoner, good input for spells.[forums.runicgames.com]
SPELLS, about Animal Handling, how works with pet and summons.[forums.runicgames.com]
PET, pet's food list[torchlight.wikia.com].