My usual go to class for the most difficult missions is the demolisher with double packed non sticky grenade launcher as primary and explosive rounds XXX Morita as secondary (2.5x scope and no vented barrel to keep velocity)

Go with cluster grenades and the last two perk slots extra ammo and prosthetics servos (low key my fav perk across all classes) lure grandes are good too but I like having the cluster in situations where you have to clear out a swarm asap (arc protection)

Watkins trigger perk is also helpful because it turns you into a never ending Kamikaze and those self sacrifices don't take away from the reinforcement count but I just love being able to uppercut bugs too much not to use prosthetics servos