Actually, I'd take Apocalypse. Physical engagement would be in Apoc's favor, when it comes to energy projection Apoc might be able to absorb what Terrax fires at him and empower himself or redirect it back at Terrax, Apoc can phase out of the way of, teleport, or morph around a lot of Terrax's terrakinetic attack, and I think he has enough offensive output to put him down, particularly if Apocalypse can disarm him.