How is it useless? I just went over this.Personally, I see no use for Diggersby it just gets trumped by the rest of the options.
It'll stay in its mega evolved state for the remainder of the battle, unless it faints and is revived. The latter of course only matters in game.Question, if i return my pokemon after Mega evolving it does it go back to normal? ( meaning i return it and can't mega evolve it again... )