Streetfighter Rogue+ Mortars (Serafen's starting and personal quest reward weapons). Is very powerful AoE DPS build. I have one designated DPS on my PotD team of 4 at the moment and she's more than enough.
Blunderbuss (mortars) modal gives you flanked, proc-ing streetfighter passive.
Only things to address for this build are: accuracy, reflex defense on enemies and suitability (minor).
Multi-class options:
Best early:
+Goldpact/Bleakwalker paladin: FoD accuracy boost, and lots of defences
Best later/with scaling (penetration issues):
+Devoted Fighter: penetration/crit damage boost to mortars, defenses from fighter tree (care,accuracy bonus from Disciplined Strikes cancels Distracted from modal)
My current/Best late game(once you get to stolen instinct):
+Ascended Cipher: damage fills your meter in 2-3 shots, so bonus damage from ascended/cipher passives and free spells
note: +1 pen with weapons, stolen instinct accuracy and defenses boost and Body Attunment armor buff/malus to enemies
Other:
+Ranger: Early Accuracy, later Bouncing Shot passive
+ Literally anything will do, most of your damage come from Streetfighter+Mortars+Distracted From Modal+Sneak Attack Proc-ed from Cripling Strike/Confounding Blind (or any other malus from your team).
Notes:
-Don't cancel Distracted with any perception buff on accident
-May work extremely well with Barring death door + Distracted from priest (see the idea from The Undying)