Doesn't know either characters so i'll have to say neither Marceline vs. Rachel Alucard from the Blazblue fighting game series in a fight to the death.
(If you don't know either character, just put "Werewolf Queen." It's better for the flow of the thread.) Marceline. Po the Panda vs Hong Kong Fooey in a sparring match.
Po Ryu Hayabusa from Ninja Gaiden and Dead or Alive vs. Ryu from Street Fighter also in a sparring match.
Definitely Twilight and Russell Fergu-whatzit. Percy Jackson and Katara to see who could waterbend the best.
Opal. Terramorphous the Invincible(Borderlands 2) vs De Roy Le(Phantasy Star Online) in a deathmatch.
Predator. Just make sure he leaves the mask ON. He is one UGLY mother-fu... He-Man vs Pac-Man in a steroid eating contest.