If you made it to the elite four but you can't beat it, then here is a good team to have for the elite four(even though it is a little team)

Lucario Level 100 + easier if Lucario has Extreemspeed, Dragon Pulse, Aura Sphere, and any other move.

Palkia Level 65 or higher + also easier if Palkia has Surf, Earth Power, Spacial Rend, and Dragon Claw

Use Lucario for the Bug Type and Psychic type elite four people and palkia for the ground and fire type elite four people. Then, for the champion, Palkia against spiritomb, and the rest use Lucario. I hoped I helped!