Once you beat the game, you'll be somewhere around level 20. You'll want to mop up kills with your guitars and then focus on this achievement as your final task.
Go to the first album and select mission 5 (the first boss fight). Pick the Grin as your guitar. Then just go to town on the boss. You don't even need to avoid attacks - you will annihilate him before he can do a thing to you. Don't forget to use Bend power.
The whole process should take about 20-30 minutes. You get roughly 1/3 a level per kill and each kill takes about 30 seconds including menu's and loading.
You can see this in action in the video below: