My highest was about 380-something, I made a build that essentially one shot most bosses; used the rounds that follow cursor, 4 barrel attachment, the barrel stock that shoots backwards(but will also follow cursor because of ammo type), 5 burst, and bullet canister.
Only downside to this build is that it lags my Chromebook and I cant see half the screen because there's so many bullets which is actually why I ended up dying (walked into spike trap that I didn't notice).