There's no real super fast and easy way to get this achievement, but reaching level 20 is pretty fast to begin with. It's the last level you can rank up to, so probably one of the last achievements you'll get. The easiest way to rank up is to complete the quests. The quests net you a lot of experience and it's a lot faster and more entertaining than grinding it out by going to the same area over and over again.
It's good to know that difficulty does affect how much experience you get when you kill enemies or complete quests. If you have completed everything but still have not reached level 20, you should bump up the difficulty and kill anything you see. The most efficient enemy to kill are Prong Soldiers. They die in 1-2 hits depending on your weapon, and give you a good amount of XP when they're killed.