Bodybuilder challenge requires you to reach certain amount of power on every ladder before promoting out of it.
This challenge requires you to be mindful of when you buy your next upgrade: how many multis do you get overall and how long do you hold your biases before dropping down to the bottom of the ladder for higher powergain.
Winning the round might be a tough task and depends on how good you are at calculating your power level at promotion. The optimal point will be promoting out right after meeting the power target.