When I used mega Aerodactyl in the battle tree, The opponent Wailren used frost breath, and the 1st 2 times it did normal damage, the 3rd time it was super effective and he fainted. How is that possible?!
Mega Aerodactyl has a x2 weakness against ice type moves. When the move roost is used, the flying type is taken away for the remainder of the turn, losing the weakness against ice for the remainder of the turn.