When you enter the arena where you have direct sight of the big monkey take a right and keep going and their will be a caveish area wishle your rexs passive hop on your yuti and go agro the king monkey. Then you run behind your rexs try to be as close to the back of the wall and wait till he is in the cave and then whistle aggressive or target. Then start courage roaring the rexs

Note: stats on the rexs should at the minimum be 30k health, 120% movement to ignore the poop, and 400 damage

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