My engy is currently level 48, and I’ve had a great time leveling her with Rifle Turret + Tool Kit to fuel Static Discharge. The last utility slot is swapped between Flamethrower, which I like for tagging mobs in DEs, Grenade Kit, or Bomb Kit, depending on my mood. Elite is definitely Supply Crate.
My main weapon is Rifle with Power/Precision focus, but I spend most of my time in kits.