If this Achievement is left over at the end of the game, you'd probably be better if you rolled over your character into a new playthrough. You keep your level, but lose some equipment. Completing the game gives you a +25% bonus XP on all future experience.The story missions also give much more XP than wrapping up side missions. The first two missions go very quickly, earning you a nice 2500 XP in under an hour. You'll also gain a new power, which can help with some other achievements.
Edit: It's been mentioned to me that, with the plentiful, good DLC, this solution is less important. I'll be leaving it, but the DLC missions provide enough XP to easily reach 30.