It's basically patience and being able to seize opportunities when you see them. Spec right, don't get cocky, learn each units attack pattern, roll a lot and again, concentrate. You need to focus because all it takes is one strike from an enemy, and you'll probably be dead. I usually go for swordsmanship, a little bit in magic and two points in alchemy (for more health). That build has served my rather well. When you finally get past Round 30, it resets so you get to breathe a little. Use the tank guy, he's amazing, though be careful not to waste all your money on pointless rounds. This is why experience is crucial. Some rounds can be beaten without the tank, some rounds you could really use some help.
So yeah. Those are some tips. And again, concentrate. Patience. That's the most important attributes to keep in mind and employ when playing arena.