Help with the first battle against primal groudon

Just got to the battle and my team is MEGA Delta Gardevoir, MEGA Delta Venusaur, MEGA Delta Bisharp, Delta Haxorus, MEGA Delta Pidgeot, DElta Ambipom and everyone gets oneshotted by the groudon. I cant leave to train up or change my team and when i get blacked out i spawn at the chansey. What can I do to continue?

You can try to reload from a backup save. If that is not possible or you don’t want to, remove Megastones from all but one of your Pokemon, since only one can Mega Evolve anyways. Equip the rest with any items you may have.