Bug pokemon vs Mewto

Although Bug type pokemons are super effective against Mewto. However, it did not resist physic moves. Are there any other suggestion besides Scizor (Bug/Steel), or tanky Forretress(Bug/Steel)? Maybe I can't use Pinsir because it's easy to get fainted. Last, do anyone suggest Double Dark moves Gyarados? Power Gyarados up?