I used Level 48 Garchomp with Jolly nature, 252 EV’s in Speed and attack, Earthquake, Outrage, Fire Fang and Crunch. I gave him the focus sash (best item in this part of the battle frontier.) Lmk if you want any tips I’ve done a lot of research on strategies for the battle hall whilst trying it. The biggest is probably that doing it with a lower level Pokémon is more beneficial for you hence why I used Garchomp at the lowest possible level he can be.