In the first level, right after you've activated the laser, just down to the level below. At this point run right and take out all the enemies, finishing with the turret attached to the box. Now turn around and run left as quickly as you can. You'll see another turret mounted on the roof above a box. Shoot this and then immediately head back right - the enemies you just killed will have respawned. Repeat this until you have the 99 hit combo.