Arena is kinda weird... you should attack when the "stakes" are at a low remaining time. That means you only have to protect them for a short period of time. You can easily defeat people with up to 20k power more than you have... it's all in your formation. Play with it. I have a low overall power, and can't seem to get higher than about 150, but I usually have about 15-20k lower power than the people I can fight. Make sure you watch the times on your stakes... if the time runs out and u don't claim them, you lose them!