For Battle Tower, they don't get good until after 50 trainers. Then they go crazy with berries that add stats after they get low HP. You could use a team full of sweepers to destroy them quickly.
A set for Milotic could be
Surf
Ice Beam
Mirror Coat(this is an egg move, if you don't have it, use something else.)
Recover/Attract/Hypnosis(Egg Move)