Use status chance incarnon evolution
Head shot + high crit + gas proc = insane aoe (1M+) that lingers for ~ 12 seconds.
The large projectile and punch-through makes it easy to upkeep Primary Deadhead.
Status Duration is VERY good for gas builds. Enemies that die will leave a gas cloud in which new enemies will walk into and die as well. Ideally you want to position yourself such that enemies are forced to walk in a tight area to get to you (survival mode, corridor, around doors, hallways)
Abuse the riven disposition, ideally multi and crit but +toxin/+status duration is optional to save a mod slot.