Don't get hit - learn their attacks, this will get you 90% of the way there
Blanks/Elder blank - Once you've learned their attacks you should have a pretty good feel for what's most likely to hit you, for most bosses this should only be a handful of attacks, with 2 - 3 blanks you should be good to go; Certain attacks when blanked will leave the boss sitting still, others will immediately allow the boss to continue to it's next attack, be aware and act accordingly.
Ironskin/Plushie/Ethereal Ring/Reloadstone/oher- items that make you immune to bullets for a period should be popped early so they can be used a second time, make sure they're fully charged before engaging the boss
Have a boss kill weapon(s) - Having a high dmg weapon (the sling is excellent) to clear quickly is very helpful, other factors include weapons with multi-hit, tracking, or bullet clearing capability are all useful for bosses. Obviously such weapons should be minimally used in room clearing and depending on your current situation/goals should be higher priority for ammo boxes