This is roughly how I'd do it. Here's an explanation of how to do it:
Create a small black platform and an enemy. Let's call the platform "GoombaTop" and the enemy "Goomba". Set the hotspot of the "Goomba" sprite to a little above his head. Set the initial visibility of "GoombaTop" to invisible. Now, go to the event sheet. (We'll call your character "Mario", who should have platform behavior, and the ability to jump, using 8-Direction Movement, or however you know how to make him jump.)
Make this event: System->Every Tick.
Make this action for the above event: GoombaTop->Set Position->X: Goomba.X Y: Goomba.Y
(This makes it so that GoombaTop stays above Goomba's head.)
Make this event: Mario->On Collision With Another Object->GoombaTop
Make these actions for the above event: Goomba->Destroy
(This makes it so jumping on GoombaTop kills the Goomba)
Make this event: Goomba->On Collision With Another Object->Mario
Make these actions for the above event: Mario->Destroy
System->Go To Layout->Layout 1
(This makes it so that if Mario makes contact with the side of Goomba, he dies.)
To make another Goomba, create "Goomba2" and "GoombaTop2", set the properties of "Goomba2" to those of "Goomba", set the properties of "GoombaTop2" to those of "GoombaTop", and create all of the same actions and events, except replacing "Goomba" with "Goomba2" and replacing "GoombaTop" with "GoombaTop2".
Hope this helped!!