How do you do THAT MUCH damage???

My build's almost finished, give or take a couple of perks. My weapons are mostly 4 stars, at least the main ones. And yet, in, say, Eviction Notice (a lot of enemies, most of them much tougher than average) I need to throw away ⅕ of my .50 cal mag to just kill one of them. Meanwhile I see these guys that are practically mowing groups of these super mutants like no tomorrow. Usually it's heavy-weapon wielding people, gat plasmas, holy fires, etc. But today, a guy with a laser rifle (judging by green beams, ultracite) was going nuts on these guys. Three seconds into a phase, every mutant spawned is dead already. I struggled to even tag them.

What can I do to have such a great DPS?