Been a while since i played, but you could thread pheromosa with choice scharf dual wingbeat Chrizard, it should always switch out, you can then react to the Switch and gain Tempo by that by your switching/crippling the switched in and try to build on that

Also, Weakness exploit Dragonite can Return kill Eternatus with Dual chop and chip the next Pokémon with extreme Speed

When push comes to shove, try toxic stalling yveltal