Easiest done in the 4th Area (Limbo)
A combo is how many kills you get without touching a platform, although, you can enter side rooms without breaking your combo, just be careful and don't touch the area outside of the time bubble that is around the entrance to the side room.
I recommend using the Armspin or the Boulder style for this achievement.
For the upgrades, I recommend having the Gem Powered and Gem Attractor. Other helpful upgrades are Popping Gems, Drone and Safety Jetpack.
As for the area itself, the only thing you need to keep an eye out is the platforms at the very beginning of each of the Limbo levels.
The enemies are pretty easy to kill, just make sure you are taking it a bit slower and not rushing through, just shoot, land on the floating objects to replenish your ammo, try not to touch any spikes and after the first 2 Limbo levels you should have your 100 combo.
For safety, be sure to cash your combo in, by landing on any surface, that being either the small edge outside of the time bubble near the side room or at the beginning of the boss area.