Master of Arms Achievement
Master of Arms25
Main character achieved level 20 as a warrior
You'll need a whole lot of experience points and you won't hit level 20 until late in the game. Kills, of course, get you XP. Do every side quest that you can. There are quite a few to be found. Keep doing them. You'll need 135,000 XP to reach level 20. This is a long grind, but you should reach level 20 before the end of the game.