The first chance you will get to "Foot stomp" aliens is in the level "The Hive", throughout this level you will come across little tiny aliens known as "Octobabies", these things will come in little groups and they're somewhat hard to see but they do attack you, so you can notice them easily. All you do to stomp on them is simply walk over them, and duke will raise up his leg then stamp on it.
The next chance you will get to try for this Trophy is in the level "Duke Burger", shortly after the beginning of the level you'll come into a room with a bunch of tiny-pig cops, some of them roaming around, some of them stuck underneath cups, so just melee the cups to reveal them. Then in "Part 3" of this level (the end, when you're on the roof) you will come across a Shrink ray just as you're about to hold off against a lot of enemies, so simply use the shrink ray to your advantage to shrink the enemies and stand on them.