THIS SOLUTION IS FOR PEOPLE WHO FINISHED THE MAIN STORY!
So I found a pretty efficient way to reach 20 level from the point when you finished the main quests and reach level at least 17.
After you kill Von Prong go up to the castle, see ending credits and than hit continue game. You will spawn at the "Von Prong Outhouse" and you will see some orgues, so kill 'em all on the path down to Orgue Village. After you kill everybody on that path, jump into outhouse in Orgue Village and teleport to the 2 random outhoses everywhere you want (thats just because you have to wait some time or maybe movement on map till the orgues respawn again), than teleport again to the "Von Prong Outhouse" (top Outhouse on right) and...orgues are respawned, so repeat this till you reach level 20. I found it pretty efficient because the orgues are on high level (17 - 18) and they attack alone most of the time, so you don't get outnumbered. I unlock this cheevo after 15 minutes from level 18, so not much of the grind.
Please comment if you give me negative vote, it helps me improve myself : )