Karl: Born Ready - Engineer - [w/ Guide] Electrical Burn

Preface:

Weapon	Upgrades	Overclock
"Stubby" Voltaic SMG	2-1-2-X-1	Turret EM Discharge
Shard Diffractor	3-1-1-1-1	Volatile Impact Reactor

Weapon	Notes
"Stubby" Voltaic SMG	Upgrades set to have ammo and increase chance of activating Turret EM Discharge. Set Turret(s) and fire at it to set off Turret EM Discharge if/when bugs get too close. Works even if Turrets are completely out of ammo, or on other Engineer's Turrets. It also ignores the armor of Praetorians and Oppressors.
Shard Diffractor	Upgrades set to ammo and Heat damage. Draw a line/circle to create magma and soften enemies; fire directly at it if the enemy is of the problematic type (flying, ranged).

Tool	Upgrades	Notes
Platform Gun	X-1-2	Repellant Additive is recommended to funnel enemies into Turret EM Discharge's range.
LMG Gun Platform	1-2-X-1	Gemini System is recommended for better Turret EM Discharge coverage.
Throwable	None	Whatever suits you-- L.U.R.E. and S.S.G. are my personal recommendations. Be careful with the other throwables as the friendly fire/self-damage can be nasty on higher Hazard levels.
Engineering Suit	X-X-1-3	Breathing Room is good to have in case you're downed and revived as the extra invincibility can let you trigger Turret EM Discharge and make whatever bugs are still around go away, or let you get away if too many strong enemies are swarming you.

The upgrades are all set on giving you ammo and chance to proc Turret EM Discharge. Due to this the Stubby has a 50% chance of Electrifying enemies and you can use Shard Diffractor's Hydrogen Rupturing (Tier 5A) for higher single target damage instead of Dazzler Module's (Tier 5C) slowing.
Set your Turrets at the desired distance and when enemies try to invade your personal space, fire at your Turrets, usually in bursts of 2~4 bullets to make sure the Turret EM Discharge will trigger and shock everything in its radius (5m which isn't too small).
Turret EM Discharge unleashes an electric AoE around the turret, yours or any other Engineer's, and the damage is pretty good and has a chance to cause Fear to enemies – 50% Fear factor, meaning it has a 25% chance to "de-aggro" regular Grunts – and it goes through Praetorian and Oppressor's armors.

The upgrades are set for area damage and ammo to put pressure on enemies or focus fire if necessary. Even without the upgrades the damage is still respectable.
Shard Diffractor with Volatile Impact Reactor is there to soften up anything not flying coming your way. Hold the fire button and draw a line or circle and anything less than a Praetorians will be pretty wounded for your Turrets or teammates to finish off.
You can fire it at the ground – same thing, draw a line or circle from floor to ceiling – while running away (or to the Drop Pod) in case you feel things are getting out of hand.
Due to its recoilless fire Shard Diffractor can also be used directly on problematic targets like Macteras, Acid Spitters, Web Spitters, Glyphid Menaces and Spitballers as well. The Heat upgrade ensures that if you can't kill you'll leave the enemy wounded enough for someone else to finish the job.
This weapon can also ignite the toxic gas from Praetorians, Neurotoxin Grenades from Drillers and the toxic spores from the yellow mushrooms in Fungus Bogs.

You can set turrets on top of platforms so they stay airborne to get the most out of Turret EM Discharge. If the platforms will be in the way you can break them.
Learning how to use the Repellant Additive upgrade will help a lot in funneling enemies and getting them in range for Turret EM Discharge.

Since being mobile is imperative to stay alive in higher Hazards, having the Gemini System helps as you can set them far from each other and run back and forth to each Turret. It's also good to have two since enemies can be so numerous they often can completely block one Turret.
Quick Deploy is equally important as you'll often be relocating your Turrets and sometimes you simply need to set at least one up pronto.

L.U.R.E. is good to make enemies stay close to your Turrets for the Turret EM Discharge and get some of the heat off you or your teammates. Since you know where they'll walk to you can set the magma from the Volatile Impact Reactor so they walk over it.
No secrets with the S.S.G., just throw them and let the damage from it and both Overclocks work their magic. This throwable also is very good to clear Swarmers and Naedocyte Shockers that get the drop on you.
Plasma Burster or Proximity Mine can be good options since sometimes you may want a "I want this group of enemies dead right now" option. They still don't make this build a good choice for Elimination or Industrial Sabotage, however.