Arenas are probably the most intimidating part of fighting in World of Mayhem. The truth is though you don't have to win in Arenas to reap the benefits.
Arenas award you with Core Boost Cubes. These are used to boost toons from each region. The difference from placing third or first from Arenas is a minimal amount of cubes. Many players don't even attempt to fight, so just attempting to fight could put you up a place in the standings and give you more cubes.
Try to get three Arenas in every day, even if you just take a loss.