Dialga

Leveling
Roar of Time - There is no reason to level up Dragon Claw, as it is weaker than Roar of Time and carries no additional effects.

Fighter
Dialga has the same ability as Palkia, meaning that you have to clear you bench before putting Dialga on your field. Dialga's Roar of Time has the unique ability that it returns figures to the state they were at the start of battle, meaning that Reshiram and Zekrom would gain Wait 9, Whimsicott loses Cotton Guard, etc. Plus, it is one of the strongest attacks in the game, competing with Deoxys EX (A) and Overdrive Reshiram for damage. However, with its much weaker Dragon Claw, completely white wheel, and extreme disadvantages with Confusion, Burn, or Paralyze, and its ability that only lets it move off the bench when all of your other Pokémon leave it, it does have several disadvantages.