Have dynamite equiped in your left hand and the chainsaw in your other. Do an impale combo on a Deadite, while his corpes is shaking on the end of your chainsaw press the left hand attack button and he will shove the dynamite in the Deadite's mouth.
The Deadite then falls from the chainsaw with the tnt logged in his mouth, when that goes it has the potenial to kill any Deadite close to him.
when your going to fight alot of deadites, use your spell called extra strength, when using this, one hit or two will kill the deadites. later on the game it won't work so well against the deadite warrior and the bigger ones.