Board Thread:Questions and Answers/@comment-32844121-20170813221800/@comment-34668827-20170814000022

Just use an IV and EV (Special Attack and Speed) Mewtwo and bring full restores, with Shadow Ball, Psystrike, Calm Mind, and Aura Sphere, use plenty of calm minds before each elite four member, then use Aura Spheres for Lorelei, Psystrike for Bruno, (If you have a Lugia use it in Bruno instead), Shadow Ball for Agatha, then random stuff for Lance, and remember to fully heal Mewtwo and Lugia in between them.

A more detailed explanation can be found here.