Not all mages can use all kinds of magic. Some focus on specific elements. Like say pyromancer.

But anon, that's where you're wrong. Serperior had access to move tutors and he was he able to get moves like Aqua Tail, Dragon Pulse, Iron Tail, Outrage, and Knock Off. He also had more egg moves with ten in total.(Why didn't GameFreak make him a Grass/Dragon) You could've made built Serperior to be a supporter, a wall, or a sweeper.