Spent all of yesterday playing 8 player smash with 8 GameCube controllers. Personally, I dont see much difference with WiiU pro controller to a GameCube controller, aside from the flipped shoulder buttons (which I remapped).
My best tip to improve yourself is play against a single CPU and learn when to attack/dodge/shield, then start adding more players in to learn how to read the flow of battle. And when you want a challenge, make teams against you and learn how to run, counter and control groups of enemies.
Or just play against people. The more you play, the more you learn.