Miltank is a pretty good tanks Pokémon, but I'd use Pangoro for the Elite 4. Fletchinder is the bird you have, and it becomes a powerful fire-flying type in its final form. Stick with it.
I'm currently training a team of mini-Pokémon that I want to fully evolve into their more powerful final forms. Murrow just needs a Dusk Stone, everyone else needs more levels.