Got to level 12 with the Worker.. I was at full health, then one shot from the lvl 2 warrior, and i was dead.
Found this tip on another site:
There is an easier way to do the Ranger's Arena.
Start as a priest, make your way up to the 12th stage along with your team. After taking care of all the ranged attack enemies, let one of your Rangers die, and grab its hat. You will turn into a ranger and you will be able to continue fighting. So, take your time and kill the other enemies.
This method only works for the Ranger though. You can tell you completed it because there will be a Ranger statue just before you enter the arena.