Make Arena power separate from real power. Every Monday at server 00:00 all players are assigned the same power level using a negative or positive boost as needed. As players win battles those with a negative boost will regain some of their power back until they reach their normal power. Loosing a battle would not reduce the power. The Arena power would not affect or interfere with the power for the rest of the game.

This will make the arena more competitive but the players that have spent time and resources to build up they heroes would still have the advantage because their Arena power will continue to increase until they reach their normal power.