Board Thread:Questions and Answers/@comment-75.97.125.87-20161212215019/@comment-32646314-20170721211654

For me what beat the elite four was the following:

2 volcarona (maxed IVs, special attack, speed, timid/modest) Quiver Dance, Flamethrower, Giga Drain and Bug Buzz. Volcarona is good for agatha (it has enough special defense to endure a few hits, 2-3 quiver dance should do the trick)

Scizor with mega stone (maxed IVs, adamant attack/def or attack/speed) Equip it with the Scizorite megastone and give it Brick Break, Bullet Punch, Swords Dance and Nightslash. Good for Lorelei, mega evolve on first turn and use 2-3 swords dance, then spam brick break.

Mewtwo (maxed IVs modest special attack/speed) Psytrike, Aura Sphere, Shadow Ball and calm mind should work. Mewtwo can be used for Bruno (you can use calm mind but not necessary), Lorelei (a few calm minds would do you good) and spam with aura sphere, Giratina on Agatha and Gary. Give mewtwo leftovers

Gardevoir (maxed IVs modest, special attack/speed) while gardevoir isn't too too bulky it should certainly do the job for lance. Give it a gardevoirite, shadow ball, moonblast, psychic and calm mind.

That was all I used to beat the elite four. Really it isn't that hard and you can use those Pokemon in combination to beat Gary (i found scizor for arceus to be LIFE SAVING)