There are a total of 21 levels in DeathSpank: Thong of Virtue, however this achievement only requires you to reach level 20. Even playing on easy, you'll be able to reach level 20 no problem. You don't get XP for is killing level 0 opponents. Do quests, and kill every enemy in your path. The best area for XP is in the Mysterious Cave in the Snowy Mountain area in the top right of the world map. You can easily gain over two levels per go in the cave since the cave is very large. It's so large, that when you're finished with the cave, you can return back to the beginning and the enemies will have already respawned.