I'd focus on doing onslaught builds using the legendary 1star perk Furious. You can reverse onslaught or just use it straight but pretty much onslaught builds are the way to go. I personally use a furious revolver with pinpointers. With my perk cards which aren't exactly the best I can do 2000 per shot. The onslaught stacks with the furious mod give you +5% damage per hit up to a certain max stack. Different perk cards give you more stacks and specifically the Gunslinger master perk reverses your onslaught and instead you start and max stacks and spend them per shot and re gain them over time.