Launcher or Mechanic is definitely the way to go for PvE.
Launchers can deal high damage easily and has very long-range. If you prefer to be more active, they're pretty good.
Mechanics basically spam robots G-series. I don't really know how they're supposed to be played in dungeons, but I'd imagine just using your G-series and spamming out whatever robots are off cooldown. V:
Mess around with the skill simulator and see what you get. I suggest looking at some videos for a general idea of what skills are good.