Jumping on people heads, jump kicking etc etc... none of this works.
You simply must get the Glopp Gun and shrink the enemies first, then stamp on them using LT (while standing).
If you missed the Glopp Gun, (as I did), It's on level 7, or 3-1 if you prefer. As you start turn left and pipebomb the gate. It'll blow up and the Glopp Gun is behind it. It does appear later in the level as well. It also pops up in 8-2, but you don't have enough shrinkable enemies in this level.
You can also find already squashable (is that a word), bugs/boars etc through the levels, these do count. There are about 20 throughout the game..