Punching enemies make them drop grenades. Beat them 'til candy comes out!
This skull can be found on the level Two Betrayals. Just before you disrupt the third relay, fly onto the plateau where the Wraiths were in Assault on the Control Room, above where you exited the underground tunnel. You will find the skull behind one of the walls there.
Piñata Skull location video here
Upon meleeing an enemy, they would drop Plasma Grenades. The number of grenades given is the number of melee dealt to the enemy.
- Piñata was a timed pre-order bonus from Amazon. It was made available to all players on the 12th of December 2014.