It would be great to have a hotkey dedicated to using a single health potion or (on a different key) mana potion. This way I could assign Q and E to those and never have to move my hand away from the movement keys. I suppose you couldn't specify which if you had multiple kinds of health potions, but if that was important in a specific situation you could still use the "normal" 1-8 keys. + if you like this idea!
Overall strategy -- I never used bombs. When you can, use the final perk ("start with gun" or whatever it's called). Prefer weapons that have a single large hit -- the SMGs can be hard to keep a steady stream going, especially against multiple AI's. Shotguns are the best, since it's easy to keep up from below (or above) someone and get a quick (and lethal) shot off. If you don't have a shotgun, rifles are good as well.
The minigun boss is actually pretty easy -- just never stay level with him. If you are always moving up and down you'll avoid his blasts. If you get an SMG, jump up to his level, start shooting and then jump up (our first shots will knock him up, so you need to jump ahead of time to keep hitting him). For a shotgun, get right under/above him and jump *right* behind him -- you can easily shoot him before he turns around and usually blast him right off the edge. For a rifle, jump up from a distance, take a shot, and go back down. No matter what, even after you hit the lethal blow, watch out for leftover minigun blasts while he flies away. I finished with three lives left after a few tries.
 "Realm of the Mad God"){kind=icon}
 "Gun Mayhem"){kind=icon}