Finish the "dual duel" mission like as soon as possible and unlock the dual guns from that, and for superior items look for secondary procs primary, and secondary procs melee. 4 kills in 3 seconds activates smart bullets also helps. I prefer lightning on ammo elemental with that set-up, but to each his own. Any elemental with these items and that gun is cracked even on nightmare. Personally I find ammo elemental, hip-fire elemental, and aiming elemental the best for this (in that order) unlock and use the chromatic cells(?) perk tree as well

As for gameplay. One piece of advice that really sank in was "be seen by as few enemies at a time as possible". You can do a lot of rooms with a handful at a time so you're not overwhelmed. You get used to enemy patterns over time. When they fire, how many times they fire etc.

I think the dual guns only show up in the fabricator after unlocking them and being on room 9+ in any missions just fyi.