When you first acquire Bolt's Laser Eyes, they're not too powerful, though they get major upgrades later in the game. At that point, the achievement is a cinch. If you just want to get the achievement done ASAP, here's a trick:
When facing low-level enemies, take their health down to where they have the "B" prompt above them for a finisher. Smack him a couple more times with a basic attack (X), then finish him with a burst from the laser eyes (LB). Without whittling down their health first, it takes a handful of shots per enemy to defeat them, and your power meter declines pretty quickly.
This way allows you to both build your combos AND finish the enemies with a laser shot while expending smaller amounts of energy.
Hope this helps.