Introduction

Turret EM Discharge is the best scaling overclock out of any Engineer primary overclock. As the modded difficulty increases (enemy spawn cap, spawn rate, diversity, health, etc...), the other Engineer primary overclocks fall behind as they start lacking in ammo efficiency and other aspects of combat. This build has been proven to work really well in both true solo (no bosco) and co-op in modded difficulties (Starship Troopers Elite, Haz5 increased enemy spawns, Haz6, Haz6x2).

Perks: Vampire, Thorns, Born Ready | Dash, Iron Will Pickaxe: Damage

Build Explanation

Stubby

• Tier 1: Increased electric chance is an absolute choice here.
• Tier 2: Both the increased mag size and reduced recoil are good choices. With reload animation cancelling, 25 magazine size is fine to work with. Sometimes you gotta do quick bursts on long-range enemies like spitters which EM Discharge lacks or you really need to spamfire your turrets immediately because the proc RNG chance is not on your side so reduced recoil is good for these. Increased mag size is nice overall especially if you're just not used to reload animation cancelling or the overall playstyle of EM Discharge, but it's not necessary as long as you know how to reload animation cancel. Honestly, taking either one feels the same to me so it hardly matters which one you pick here as you can easily adapt your playstyle to either choice.
• Tier 3: While EM Discharge already has extremely good ammo efficiency, more ammo is still better than the damage option. Stubby with EM discharge already has pathetic direct damage, +2 more damage isn't gonna significantly change that. And this is Turret EM Discharge, not a direct dmg build. You'll kill more enemies and get more in return from having +120 ammo instead.
• Tier 4: By nature of the EM Discharge playstyle, you'll make use more of conductive bullets.
• Tier 5: Electric Arc is extremely good and it's your way of dealing with enemies that are out of range of your turrets. If you need mag size, get it from T2 instead.

Breach Cutter

My secondary, Breach Cutter, build is the same across all of my Hazard 6x2 Engineer builds. I think Breach Cutter is insanely powerful because it provides both good AOE and single target whereas the PGL can only either provide and specialize in one but not the other. In Hazard 6 and up, it becomes increasingly more important to have both.

Overclock

A lot of the overclocks that Breach Cutter has are good. I personally prefer Inferno because I mostly play modded difficulty with players that are centered around a fire team composition. I still use Inferno though even on solo because the fire status effect is really good and scales really well because it can spread. Not only that, with Inferno you can one-shot spitballers. However, even if you choose to use another overclock, the build setup that I use is still good for other overclocks.

Tiers

• Tier 1: Having increased projectile lifetime means that you will have longer range. This is in particular synergizes really well with T5 Plasma Trail. With the Inferno OC, I rarely shoot more than two breach cutter lines at once, most of the time I only shoot one breach cutter because I want to take advantage of the both the fire DoT and the plasma trail DoT as much as I can. The increased range is also very helpful for sniping spitballers and breeders, for which there is many of in Hazard 6 and above modded difficulties. However, increased magazine capacity is also a decent choice because it effectively gives you 3 more ammo. It also allows for some more burst DPS, so in particular its nice against the dreadnaughts or to just quickly fan out an area of enemies in all directions.
• Tier 2: +6 ammo is way too good to pass up on. The other two mods can't really match its value.
• Tier 3: Quick Deploy is good so that you can actually use your Breach Cutter if enemies get really close on you, especially in Hazard 6/7 where enemies are way faster. Since I rarely fire more than two breach cutter lines, I typically don't manually reload the Breach Cutter and just rely on Born Ready perk to reload for me. Even if you don't use Born Ready, you can do reload animation cancelling so you don't really need the faster reload mod on this tier.
• Tier 4: Stun mod. Engineer has the worst survivability relative to other classes, you want the CC here.
• Tier 5: Plasma Trail for extra DPS and this mod is really good for swarmers/jellies/naedocytes. Honestly once you get used to aiming with one line, you really don't need Triple Split even against mactera. Against mactera, they typically go into a thin and straight formation that makes it easy to wipe all, if not most, of the mactera all at once even with a single breach line. However, I do recommend using Triple Split Line if you're playing the Mactera Plague warning.