Reached Level 20.
This is unlocked immediately upon reaching Level 20. You are not required to spend the points you have earned for the achievement to unlock. Although not needed to obtain this achievement, there is a glitch that can be used to obtain experience and money very quickly. This glitch can be performed using any side-quest that doesn’t involve a cut-scene to turn in. Save your game before talking to the NPC required to complete the quest. Draw your weapon with and as you are putting your weapon away talk to the NPC. Continue spamming to earn continuous amounts of XP and money. A video demonstrating this can be seen below.