Adam: The easiest way is to have a pistol or submachine gun, and a katana. Stay back until he begins to blow up his balloons and shoot them while he's blowing, it will pop and he will be vulnerable to hit with the katana, a few tries should kill him, after he's dead every time you go to wonderland plaza a chainsaw will be there

Larry the Butcher: He is very easy, just get one of the toy chainsaws from Adam the Clown and as soon as the cut scene is over for the butcher just run up and slice him, it should kill him in about 5 seconds if you do it fast enough

Cliff the Machete Man: Once again, use the toy chainsaw and cut him until he jumps in his underground tunnels, then run out and run back in, he will be standing right in front of you charging, just hit him again, repeat this process (note, when you go in and out his life will not change)

Cletus the Gun Owner: This is probably the hardest boss on the game. To kill him, it takes a little bit, you can try to hide behind one of the two shelves as soon as you walk in but if you ask me the easiest way is once again with the chainsaw and brute force, just run up to him, don't jump over the counter or he will throw you out, and just keep trying to slice him, the best time to do this is when he is taking a drink of his wine

Steven the Grocery Maniac: This guy is pretty simple, all you really have to do is as soon as it starts run and hide in the aisle right beside the one your in, and as soon as he comes around the corner to look for you slice and dice with the katana or the chainsaw

Sean the Cultist: This dude can be sort of difficult. He loves to chase you down and slice the crap of you with his Excalibur. He is extremely fast and the best way i think to beat him is with a katana or once again a chainsaw (they can beat any boss with ease). Wait for him to do his jumping swing and as soon as he lands if he doesn't hit you, hit him as much as possible, it should be over after a couple tries.

The Convicts: A lot of people say the sniper and the submachine are the best but if i were you i would check this cheat out. If you are a decent freestyle aimer with a shotgun this should be no problem at all. Just wait until they hit a tree, and you have to shoot them without holding RT because otherwise the .50 cal guy will shoot the crap out of you. Just go up and shoot the guy shooting the gun first; it will only take about three hits. Then try to sneak the .50 cal off the gun and it is very simple after that. I can take them in about 45 seconds doing this method because of how powerful the shotgun is against them.

Additional Tips:

1) The toy chainsaw has to be the best weapon to face bosses with because it is a relatively fast weapon and it is extremely powerful.
2) If you can, practice hand to hand combat moves the most because that is the ultimate thing you can ever know, it has gotten me out of so many tight spots, it is probably the most effective way to kill a zombie in one hit. The somersault kick, the knee stomp, face crusher, and the double lariat are the moves you really need to master because they are crazy good.
3) Try to avoid the cult members in the yellow jackets at all times
4) This may sound really weird, but one of the most effective weapons in the game is a simple hunting knife. It kills spec. Ops guys in only four hits and all zombies in 1 or two. It is extremely fast and can also take down almost any boss.
5) This one is just for fun but get a soccer ball and aim it straight up and push X, watch this thing fly
6) I know most cheats don't work believe me i tried a few of other peoples and they suck, my method is faster and they work.