Speedy Engie

Engineer has two major weaknesses: Self Defense and Mobility.

The first is a skill issue, and this build fixes the second.

The goal is to make a loudout which makes you more flexible as a class. Other builds are stronger at Engineer's specializations (controlled waveclear, area lockdown, and even some single target builds), but this one tries to be able to do it all a little plus some things other classes are meant to do.

Starting with the Warthog running 12321. Because it has such a good reload cancel, (quickly tap melee or pointer right after engineer actually loads the ammo) magazine size isn't as necissary as you would think, so we take fire rate here to offset Magnetic Pellet Alignment's downside. Next we take pellets, since the extra damage is extremely useful for making grunt and mactera spawn oneshots more consistent and we should be ok with both ammo and spread if we use our tools wisely. Tier three is honestly a toss up. None of them are especially impactful, but for my money the magazine size is best just for convienience's sake. Feel free to take choke or reload here if you like, it's by far the most flexible slot. Tier four is way easier, armor break is terrible and damage is very good, we take damage without so much as a thought. Finally, turret whip is crazy strong and the primary reason to use the Warthog, take it and cherish it.
Magnetic Pellet Alignment simply makes the base shotgun a better weapon. You can actually use it against enemies not within petting range and grunt oneshots to the face are way easier with the damage up. The fire rate downside also doesn't feel nearly as bad as it looks, so you end up with a versitile primary that can act as waveclear with turret whip and has decent single target damage and stun. Not too shabby.

Next is Deepcore, which is the heart and soul of this build. 11132 is my prefered build, but honestly as long as you take heat in tier three and stun in tier four everything else is up to you. The only thing I would keep in mind is that tier one range is a little better than it looks because it doesn't do less heat damage near the edges of the range, and range makes your RJ250 jumps stronger (more on that later). Stun is just universally useful on a splash damage weapon, and works well with heat since enemies dying via fire take a bit longer to go down, so slowing them down is nice.
But that's all boring stuff, what you're really here for is the star of the show: RJ250 compound. The extra ammo is nice, but it's the blast jumping that changes the game. Shooting at a surface near you (mostly the floor, but sometimes a nearby wall) will propel you in the oposite direction, turning you from the slowest class in the game to the second fastest behind Scout. You know all those times you see minerals up high but your Scout isn't anywhere to take advantage of your platforms? Take advantage of your platforms yourself! Need to cross the caves quickly, blast jump will do it! You're surrounded and can't kill all of them? Blast jump away and deal with your problems later like a real dwarf! The sky (or rather, the cave ceiling) is the limit, and this weapon scales directly with your skill in using it. Here's some tips for the greenbeards:

1. Run dash. You keep your momentum when you're blast jumping, so if you do it right after a dash you go just about twice as far. Dash is also just a good perk in general, but this pushes it over the edge.
2. Run born ready. Being able to switch away from the Deepcore right after a launch and still know you'll be able to use it when you need to next is hugely impactful, and being able to be confident that you have platforms in the clip for your next jump is a gamechanger, speaking of...
3. Get good at landing on your plats. Seriously, the fall damage reduction from the T2 platform upgrade is crazy strong, and basically means you'll survive just about any fall. Being able to place platforms where you expect to land makes you able to take far riskier jumps, and even escape danger better since you can jump and place a platform to land on attached to a nearby wall to immediately have the highground and be clear of bugs.

Other than that, this build is pretty standard. 212 is the best platform gun build since the ammo is nice and the other two T1's don't do anything for you, and T3 Repellant is very useful for controlling the movement of bugs. Just placing a few plats behind you will usually be enough to prevent flanks, which makes you signifigantly safer. Turrets run 12x1, with me prefering 1211. Gemini + Defender has great synergy with turret whip Warthog, and being able to build them faster really enables the speedy style that RJ250 likes to play with. I chose armor breaking in T3 because it basically acts like a +10% damage upgrade to the turrets a lot of the time and against larger enemies can make it easier to hit your shotgon shots, but stun for better protection and bigger expanded ammo for less babysitting are both completely reasonable options. 2113 suit is probably best, since you want more and faster regening shield for more frequent blast jumps and breathing room can often be the difference between winning and losing a mission. Pickaxe is personal taste, but I like the cooldown since it helps both with mining and with using the pickaxe stun as a self defense tool.

Finally, shredders are my preferred grenade for this build just because of how stupid easy they are to use. Whenever you're overwhelmed or feel like you're going to be soon, pop a shredder and it just makes life a little easier with a slow and damage over time to a few enemies around you. Plus, almost more importantly, you can hear when and where shredders are attacking, which can often give you a sort of bat echolocation to bug locations. It can clue you in to when enemies are around a corner or coming from behind, plus it deals with swarmers and the little flying jellyfish guys extremely well, which is hard for this build otherwise without turrents. It's just a nice compliment to an already flexible build.

Have a pleasant flight, Rock and Stone!