In a word, Parkour. It’s a useful skill for Engineer, Gunner, and Driller. Even as a Gunner you cannot count on having a shield charged and available every time bugs get close enough to nibble your butt so you need to learn the skills of using movement and terrain to keep the crawlers at bay.
For example you can hop a gap, bugs cannot so they have to turn around and path-find a new route to reach you. This affords you the time to prioritize targets. Parkour away from some, while focus-firing the ones that are the immediate threat. (I.e. fliers, spitters, and whatever is closest) Where the game doesn’t provide you with terrain to parkour between, the engineer is equipped to make their own terrain.
This gives the engineer the ability to be one of the most untouchable classes in an area, especially given that platforms can negate a good chunk of fall damage where a bad grapple can kill or cripple a scout.
Engineer’s starting weaponry is lacking, and only really starts to shine with overclocks, with exception to the Breach Cutter and Shard Difractor, Those things are a beast without OCs. the PGL is also decent fully upgraded when set up for area damage and incendiary. While Gunner has the shield as a panic button, the Engi does not, but he does have LUREs as well as Shredders which are very useful for getting out of a sticky situation. An engineer’s strength though is in preparation.
Get your turrets down early and covering an area you are working. They can easily kill/stun stuff sneaking up on you, and act as a doorbell to know something is approaching. An engineer covered by his turrets has little to fear from Exploders while he’s mining, or getting harassed by swarmers or zappers. They are a good warning for mactera as well.
When first starting off with the Shotgun and PGL, you’ll want to make full use of LUREs to occupy big targets like Praetorians and Oppressors to get close and shotgun their butts, then spec the PGL for area damage and proximity fuse plus incendiary to take care of groups of trash. A lot of the time I see Engies fire their base PGL at the biggest target which doesn’t do a lot. 1 PGL round can take a sliver off a Praet, or it can kill 8 grunts. Use it on the groups. LURE is still one of the best grenades in the game, and certainly the best starting grenade of all classes.
Don’t drop it at your feet, toss it to a location considering where the enemies are and which way you’d like them to be facing. This can save you tonnes of ammo. If you have Mactera swarms the LURE can save your life, toss one, then put a platform over it. This will prevent the mactera shots from damaging it, allowing it to last a lot longer.
The shotgun and stubby really shine when you integrate them with your turrets with Turret Whip (No OC required) and EM Discharge or Turret Arc. (OCs) For instance a savage turret whip build can be done with an auto shotgun with mini shells OC. Or you can exploit the stun aspect of the turrets with Stunner, or turn the Shotgun into a mid-range weakpoint murder-stick with Magnetic Pellet Alignment if you’ve got aim down.