EDIT: If you can't unlock the achievement playing on the main story, do as LeaoSomogyi says and try on Trip's story, playing the same level.
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This is easily done with the boss on Lagoon City Zone Act 1.
This boss spawns pond skaters enemies, and if you hit the boss more than 2 times, it will stop spawning enemies.
As Trip, tap
for jump and push
again (and hold) for the second jump (when she looks like a donut with spikes)
Every time that you defeat an enemy, press and hold
again for the second jump and hit the next enemy (as a donut)
The achievement unlocks when you defeat the 100th enemy