Unfair Colosseum Battle

You touch the ground, creating a colosseum to eradicate your opponents. The colosseum is located in a demiplane created by the spell. Pick any targets within sight to enter the arena. The selected targets will teleport onto a central arena which is a 500 foot diameter circle. The targets not selected will teleport in as spectators. As a spectator, you are under the effects of the Invincibility spell until you leave the Colosseum.

There is an impenetrable magic barrier seperating the inside and outside of the Arena for the purposes of deterring escape and ensuring the safety of the spectators, as well as preventing spectators to alter or damage anything inside of the Colosseum.

You may decide the arena type and enemies within the Colosseum:
Arena types:
Sandy. The entire 500 by 500 foot arena is covered in sand. This sand can be affected by magic, and can be hardened with water.
Forest. The arena becomes a forest, with many trees. There is a clearing in a 200 foot diameter circle at the center of the arena.
Ocean. The entire battlefield becomes a 50 foot deep ocean.
Rock. The arena is covered in sharp rock that make the entire battleground difficult terrain. Additionally, falling prone on the sharp rock will grant you the prosperous reward known as 2d6 Piercing damage.
Custom. Feel free to make your own Arena, because the spell is infinitely customisable.

Enemy types:
I'm too lazy to add this part, so please refer to the graph below for the spell level.
Level 6: Up to 10 CR 3, 3 CR 6, or 1 CR 11 enemy.
Level 7: Up to 25 CR 3, 4 CR 7, 2 CR 11, or 1 CR 13 enemy.
Level 8: Up to 50 CR 4, 5 CR 8, 5 CR 11, 2 CR 13, or 1 CR 15 enemy.
Level 9: Up to 50 CR 5, 8 CR 10, 5 CR 15, 2 CR 18, 1 CR 25, or 1 God (you must pay a fee of 100,000 GP to summon the god)

The spell ends when all the creatures on one team are dead, incapacitated, or forfeit.