When you are close to level 29, make a new save. Earn some XP to reach level 29. When you hit 29, use the Here and Now perk. This will immediately make you level 30. For the level 30 perk, select Devil's Highway. This will make you very bad and the achievement will unlock.
A good way to get some quick XP is by collecting Super Mutant blood packs for Tristan. After completing Who Dares Wins, speak to the Paladin in the Citadel and he'll set you the mission. Collect blood packs from Super Mutants and take them back to Tristan. Two good spots for Super Mutants are the Germantown Police Station and the area surrounding the Washington Monument. Be sure to be well rested, and you'll recieve an XP bonus when handing the blood packs to Tristan.