The best way to beat that evil clown boss near the roller coaster is as follows:

1) Go get about 4 pistols from the fat zombie guards in that area of the mall. A container of OJ is good to have as well.

2) When the fight starts, run back down the stairs to the upper level and KEEP YOUR DISTANCE from him. He will start to blow up balloons of gas when you're far away.

3) Shoot him as he is blowing one up. The clown will drop the balloon. Shoot it while it's near him. He'll start coughing. Shoot away.

4) Repeat until he's dead. Watch out for zombies while you're fighting him, and NEVER try to melee him. After beating this boss, he'll leave a small chainsaw behind after he's dead. Pick it up before you switch off the coaster, because after the cutscene you'll find another at his body!

5) Final note, the hostage you save should be saved if you want to get the best ending. This hostage unlocks a "warp" between the Wonderland and Paradise bathrooms, letting you quickly move from one side of the mall to the next.