I used Mirror Coat Magnezone at first, but then I thought, "Eh, that was too boring." So I soft-reset, and put Blissey in the front.
Took a Focus Blast, set up a Light Screen. Took another Focus Blast, recovered health with Soft-Boiled.
Took a third Focus Blast, Soft-Boiled again.
Took a fourth Focus Blast, hit it with Toxic.
Made him use a Full Heal, hit it with Toxic again.
Once I stuck my toxic poison, I just stalled it with Light Screen and Soft-Boiled until it fainted.