First of all, you need to choose the path that has a flying robot (drone) next to it at the beginning of the run.
Now, you need to kill a hundred chickenlegs (The robot with legs). Other enemies do not count towards the achievement but you will need to kill most of them anyway to progress through the levels.
If you finish the fourth level with a x10 combo or higher, you will enter a bonus fifth level with tons of enemies. Aim for that to earn some bonus chickenleg kills during the run.
After you finish the last level (whether it's the 4th or 5th), press
and choose DEATH'S LAIR
to start another run.
Repeat till you reach your 100th kill and the achievement should unlock immediately.
Sometimes the progress of the achievement gets stuck at a certain percentage. If that happens to you, then you will need to get more kills than what the percentage suggests. In my case, the progress bar was stuck at 85% and it required around 40 more kills to start registering again. Worst case scenario, you start from scratch.