Blunk biting Cedric's tail in the season one finale, finally showing his own action chops.
Blunk: Blunk not scavanger. Blunk warrior!
The So Last Season battle in "J Is For Jewel," with the girls completely curb stomping the guy who gave them so much trouble in season one.
Matt freeing himself from Shagon's control. First, he convinces Shagon to give him brain and body control to sing a song. Just when Shagon's about to kill Will, Matt takes over and starts singing, tipping off Will and cutting off her hatred so that he can finish Shagon off. You wouldn't expect a guy like Matt to be a Guile Hero, would you?
Matt: Game, set, match, Shagon. So Shagoff! *blows him to bits*
Will versus Phobos at the end of the Trial of the Oracle arc. Phobos, using his status as temporary Oracle, subjects Will to a Mind Rape to try and convince her that if she doesn't surrender the Heart of Kandrakar she'll end alone and abandoned in a Bad Future. Will falls for his lie... And then proceeds to try and disintegrate him, stopping only because Phobos had pulled a Grand Theft Me with one of the Wise Ones of Kandrakar and she had to fix that.
Their following encounter ices the cake: Phobos is rightly worried when he's left defenseless and the Guardians have decided to take no more chance with him, but what makes him cry "Nooo!" in holy terror is Will's demand that she will give the killing shot. Quite justified, given that Will ended up disintegrating his soul...