How can I kill Evil Marie or Krampus on one life?

Having just achieved the world record, I can confirm all the bosses are killable in one credit.

http://www.aurcade.com/members/submission.aspx?id=169142

Krampus: I aimed for the upper chest, just below his chin. That guaranteed a reasonably equal shot placement between his head, chest, and arms. Bloodying each of those areas causes him to stagger, which buys you valuable time to get additional hits in. Shooting the legs staggers him for a little longer but I personally had a lot of trouble getting enough focused shots in to get them bloodied during the heat of the fight. He doesn't take damage while he's skating, but he does take hits on the throne for a second before the fight starts. If you shoot fast enough, you can even stagger him before he jumps down to attack.

Also, the initial question was incorrect: you can't get to Krampus with full health. Since you're required to take half a bar of damage from behind at the garage shortly before the fight that means you have to fight with half a bar of health at most.

Evil Marie: She's harder than Krampus because she lacks the big beefy hit areas, but since you're not required to take a serious amount of hits in her stage and there are three health pickups as opposed to Rickety Town's one you can enter her arena with way more health so it ends up being a lot easier to deal with her.

Just start the fight shooting as quickly as possible into her chest, after enough shots she staggers a little bit. I found you get a good bit of extra damage by going easy on her in the beginning section of the fight and just shooting her until the bar drops to the beginning of the last 'E' in her name, letting her fly into the air for the axe throwing section, and then shooting her extra while she's on her way up into the air and during her teleportation back onto the ground. If you shoot too fast in the beginning she skips flying up to throw the axes and you have less time to do the required damage. Unlike Krampus, she has no invincibility during movement so you can do a ton of damage by shooting her while she's on the bridge. She technically has a secondary axe throwing stage on the bridge but I always out-damaged her and forced her into the energy ball throwing section.

At the end of the day, the solution to CarnEvil really is just to up your shorts-per-second enough to out-damage the bosses. I ended up bracing the gun against my chest, pumping with my left hand, and feathering the trigger with my right hand while shifting my chest to aim broadly at different sections of the screen.