Sweet game, my man.
What you do first is beat the game on easy which will unlock the Revolver. Here is how you beat it ON EASY:
First Boss: You do not need my help on this one. At all.
Second Stage: On the second part where the zombies are attacking, shoot at the red splotch on their shirts that is where the "hole" is located, NOT at the rip towards the bottom. When you are fighting the boss only kill TWO of the Zombies. With only one zombie left the Count can't call in more Zombies constantly. Keep shooting at the Count and reload when he goes off-screen. Keep in mind that he is temporarily invincable when he is putting his wings on. But when he is flying you can still hit him. Hit the beanie on his head to knock him down when he is doing that DBZ blast thing to knock him down and get easy hits.
Third Boss: Shoot at the Minigun whenever it appears and keep your focus on the two tanks next to him. Keep shooting at those until they both EXPLODE, do not start shooting at the boss just because his shield goes down. Once both tanks are down just keep shooting at the heart until he goes down.
Fourth Boss: When he runs across the room blast him so he stands still and laughs at you. While he is blocking/laughing shoot the chandelier down so it will pin him to the floor and shoot him while he is knocked down. Rinse. Lather. Repeat.
Final Boss: On the first part make the little gnomes your main priority. Only when there are no Gnomes in the room should you shoot at the King. Trust me, you'll need the health. When you are on the second part it's mainly straight forward. His gun is like a shield so I suggest just shooting at his head. Shoot his attacks whenever possible to minimize damage and you should come out ok.
ONCE YOU BEAT EASY
Ok. now where the REAL FUN starts! There is a glitch in the game where if you switch weapons the ammo fully recharges so you don't have to wait for the reload! On Medium keep using the Revolver and switch to the Pistol and switch back the Revolver to regen it's ammo. Only use the pistol to take down the weak points on zombies. Once you unlock the nailgun use that to take down those pesky zombies instead of the pistol. I found it to be better at hitting the weak-spot. Now that you are on hard use the same Switch-to method with the Revolver and the Nailgun and use my strategy to get to victory
NOTE: On the second boss it is recommended that you get good at blocking the Zombie's Leg attack to once again minimize the damage you take.
Now that you have beaten hard mode enjoy the best (and sexiest) end picture after the credits!