Really easy - you will come across old looking dudes carrying wooden shields early in the game, on the first level in fact.
About 1min into the first level, you'll see two guys with shields, and you're above them. Simply walk onto one shield, wait for him to punch at you, press D-Pad down and X to kill em.
Another level is the second level of Teensies in Trouble: Creepy Castle. Run up to them and land on their heads, they will raise their shields to protect themselves. You are now standing on the shield, wait for them to punch at you. Press D-Pad down and X to kill em.
Bloop - 10GS