Nothing particularly complicated about this. You can do sidequests for experience to help you reach level 20 but there is a much easier way to get experience. Near the end of the game you'll find a dude in a cage after the area Valhalla Heights (trying to avoid spoilers here). He will ask you to kill a bunch of enemies. In order to progress through the quest, you must kill the enemies while they are on the pentagram symbol. If you don't, they don't stop spawning, so you can kill as many as necessary to get to level 20.
Which is to say, don't bother spending time leveling up unless you have to in order to progress. I went from 18-20 in about 5-10 minutes this way.