re: To anyone who's played The Division

I don't know which Cleaner boss you are talking about, but take out his lackey's first and then just keep your distance from the boss. Their flamethrowers have a good amount range, as already mentioned shoot his tank, that'll knock his health down quite a bit, just keep your distance, deploy a turrent (shock) to stun him for a short time and use that opportunity to bring his health down more.