Karl: Born Ready - Engineer - [w/ Guide] Plastcrete Mayhem

Preface:

Weapon	Upgrades	Overclock
"Stubby" Voltaic SMG	2-3-2-X-1	Micro-Conductor Add-On
Shard Diffractor	3-1-1-1-1	Plastcrete Catalyst

Weapon	Notes
"Stubby" Voltaic SMG	Upgrades are set to have more ammo and compensate for MCAO's (Micro-Conductor Add-On) penalty. Set a few platforms around the area and electrify them to trim down the swarm. You can also electrify two turrets to create an electric fence, but beware this has self-damage and friendly fire.
Shard Diffractor	Upgrades set for ammo, extra weakpoint and Heat damage. Use it to explode platforms with Plastcrete Catalyst if a problematic enemy is close to it. Exploding platforms have self-damage and friendly fire damage, so very careful with it.

Tool	Upgrades	Notes
Platform Gun	2-1-2	Upgrades set to have as much ammo as you can. You'll be using platforms to create shock and explosive traps against enemies.
LMG Gun Platform	1-2-X-1	Gemini System allows the old Turret Arc to be enabled, but beware of the self-damage and friendly fire it causes.
Throwable	None	No wrong choice here, but L.U.R.E. and S.S.G. are my choices for this build.
Engineering Suit	X-X-1-3	Breathing Room is good to have in case things go wrong.

Upgrades set for fire rate and magazine size to compensate the penalties of MCAO
Stubby should be used to electrify platforms which will slowdown and damage incoming enemies. While it won't kill anything tougher than a Grunt Guard on its own, the damage it does and the breathing room it gives, especially in cramped spaces are two things to consider.
Alternatively you can scatter platforms and electrify them as you run to shock pursuers or deal some preventive damage to them.
MCAO allows you to electrify two turrets, either with Gemini System or other Engineer's turrets will create an electric fence. Grunts, on vanilla Haz 5, die before being able to cross it entirely while Slashers and Guards take a considerable amount of damage. Even Praetorians will take a lot of damage if they get stuck in it. Just remember that the electric fence of MCAO has friendly fire.

This weapon has no recoil and basically infinite range + hitscan, use it to bring down any problematic target, mainly ranged ones, that are distant.
Plastcrete Catalyst is there to explode any platform and wound tough enemies like Praetorians. It can also kill groups of Grunts handily, but BE VERY CAREFUL WITH SELF-DAMAGE AND FRIENDLY FIRE.
The Plastcrete Catalyst explosion ignores armor; it can wound Oppressors and is big enough hit the Hiveguard's "horns", all of them if the Hiveguard is close enough to the platform.

Ammo is the choice here since you'll be spreading a lot of them for MCAO and Plastcrete Catalyst. Repellant Additive is NOT recommended for this build.
Spread the platforms in places where you think bugs might walk to. Make sure not to overdo it, though. Always have a few for emergencies.

Gemini System is used to create an electric fence between the Turrets within 15m of each other. However, like Scout's Electric Bolts, electrifying one turret doesn't cause friendly fire, but the electric fence from two turrets does have friendly fire, so avoid using it in cramped spaces.
Mk. II can be electrified, but you won't be able to create electric fences (unless another Engineer builds one) and enemies will need to get very close (2m) for the turret to shock them.

L.U.R.E. is great to make enemies go and stay near your platforms. You can shock them with MCAO or explode it for higher damage with Plastcrete Catalyst.
S.S.G. is a general purpose throwable. It'll help chip away enemies slowed down by MCAO and get rid of pesky Swarmers or Naedocyte Shockers that get the drop on you.