Arkedo Series

You kill enemies by jumping on them or by roaring at them ( when your roar bar is full). The kills are cumulative and unlike collectibles, the kill count is saved even if you die before reaching a checkpoint. If you have not unlocked this trophy after your first playthrough, you can either select levels and play them again or use the farming spot mentioned by Puzzler:

In Level 2 (Caves), after the first checkpoint, 5 duck enemies will be conveniently placed in close proximity to each other. Use these enemies to fill your roar bar, and progress past the signpost that reads "New weapons have been shipped! That dirty cat will no longer jump on our heads!". After it, you will see 8 mushroom enemies lined up in a row. Roar at them then kill yourself.
Rinse and repeat until you get the trophy.