Hazard 6x2 Warthog Engineer

This is the Warthog build that I use on both solo and co-op Hazard 6, x2 Enemies with great success. Here's a video of me using this build on solo Hazard 6x2: https://youtu.be/GVuZR9O2mWY

Check out my other Hazard 6x2 EM Discharge Engineer build: https://karl.gg/preview/4798#/

Warthog

T1 - You already have low RoF with this build from the downside of MPA + choosing T5 Turret Whip, you want the RoF here. T2 - Both ammo or loaded shells are good picks. I personally rarely have trouble with ammo; when I was originally using T2 ammo before, most of the time I was resupplying with over half ammo left on my Warthog hence why I didn't need this. Sometimes even with just 90 ammo, I'm still resupplying with slightly over half ammo left on the Warthog. Therefore, I chose loaded shells because it increase your DPS and indirectly buffs your stun chance. The reduced base spread makes the least sense out of the three; MPA already reduces your base spread. With 50% Base Spread, you can already reliably 1-2 shot long-range spitters on the ceiling. T3 - Nothing really stands out here. Magazine Size makes the most sense, especially if you already know how to reload cancel so you don't need faster reload on this tier. However, taking faster reload isn't a bad choice either. You could also go with reduced recoil but due to the low RoF of this build, recoil isn't really a big problem at all. Any pick here is ok. Won't affect you that much. T4 - Bigger pellets. T5 - So taking one or the other won't really hurt you at all. Turret Whip is really good, but from my experience, it's hard to be consistent with it on co-op at least. Regardless, it is better than nothing and I advise you to go for this as your first choice. I personally don't mind the low RoF of this build so I do not need the T5 RoF, but if the RoF is too slow with this build then taking Miner Adjustments is not a bad choice at all. So pick Turret Whip unless you really cannot stand the low RoF of this build.

Inferno

Contrary to the common opinion, Inferno is actually a really good overclock for the Breach Cutter. In modded difficulty, the fire spread mechanic is extremely powerful so just the fact that it lights things on fire makes it a better choice compared to the other BC overclocks. This build allows you to one shot spitballers provided you wait for the DoT to tick down. But if you don't like Inferno, then you can basically use this same exact build but just switch out the OC with your preferred choice.

T1 - Lots of spitballers and breeders in Hazard 6, you want to have long range capability with Breach Cutter. Having higher magazine capacity is not a bad choice if you're using Light Weight Cases OC instead. T2 - No other really solid choices besides ammo. T3 - Quick Deploy is nice because Haz6 enemies are a lot faster and it's good to pull out in a pinch. Born Ready perk and reload animation canceling get rid of the necessity for the reduced reload mod. I very rarely manually reload Breach Cutter, most of the time it's my Born Ready perk that reloads it instead. But if you don't take Born Ready with Engineer (which I highly suggest you to take it, Born Ready is amazing on Engineer especially since he tends to use all of his kit at once in a single wave), then maybe faster reload isn't too bad. Although reload animation cancelling still gets rid of the need for this. T4 - Stun. Out of all the classes, Engineer doesn't have any quick, reactive, and close-ranged defensive capabilities. Gunner has shield, Driller has drills which has fear and stun, and Scout has grapple. If backed into a corner, Engineer suffers the most. Therefore, stun on BC helps a lot with this. T5 - If you're doing Mactera Plague, the Triple Line is good for that. Otherwise, Plasma Trail. Good for lingering swarmers/shockers/grunts. And overall increases your dmg just a bit more.

Turrets

I run max ammo turrets because your turrets + breach cutter is pretty much your main wave clear. Honestly, with any build in mind, your turrets + breach cutter is actually like 70% of your firepower. Try this in Haz5: play Engineer using only nades, turrets, and secondary as your damage; no primary weapon at all. You'll see that your primary doesn't make that big of an impact when it comes to wave clearing. This is also why I don't need ammo at all with the Warthog. Therefore, I go full ammo with the turrets taking T1 Gemini, T2 Ammo, and T3 mag size.

One downside of this max ammo build is that you have to be a lot more proactive in placing down your turrets. 2 vs 4 seconds construction time is quite a significant gap, especially if you're trying to build as a wave/swarm spawns. This downside does make playing Morkite and PE missions in particular a bit more annoying. In co-op this isn't too big of a deal because you have teammates, but in solo you have to be more thoughtful of anticipating when you need to place your turrets down.

Grenade

All three of Engineer's nades are all good choices. I pick Plasma Burster to combat the turret downside of slower build speed. Since you typically don't want to constantly move and re-build your turrets, having plasma bursters is a good defensive tool to have for whenever you are out of your turret range. It does really good damage, you can carry 6 nades, and it fears too.