Some Toxic users, Steel Types, Haze/Phazers, Faster Pokémon, Physically Offensive Pokémon, Physical Walls, Physical Tanks, and Revenge Killers Take Mimikyu out easily.
Consider using Lopunny-Mega, Medicham-Mega, Scizor-Mega, Skarmory, Bisharp, Tangrowth, Heatran, Landorus-T, Toxapex, Magearna, Celesteela, Ferrothorn, Clefable, Mawile-Mega, Tapu Bulu, Kartana, Alolan Muk, Bisharp, Magnezone, Gliscor, and Pinsir-Mega. None of these are guaranteed to work, but they can take down Mimikyu pretty nicely, some needing Mimikyu's Disguise to be broken, however. This could also vary by Format. :P