From Cervantes De Leon | 9th Jun 2009
The 11 executions are as follows:

1. With any weapon equipped, pick up a downed enemy with A, then snap their neck with X.
2. With the Sniper Rifle equipped, press Y to use it like a sledgehammer on the opponent's back.
3. With the Sniper Rifle equipped, press B to slam it down on the opponent's head.
4. With the Torque Bow equipped, press B to nudge them with the Torque Bow's blades.
5. With the Torque Bow equipped, press Y to place their head between the bow and the blades and kick for a bloody decapitation.
6. With the Boltok Pistol equipped, press Y to slam the gun on the top of their head.
7. With the Shotgun equipped, press B to hit the opponent's head, using the shotgun like a golf club.
8. With the Boomshield equipped, press Y to slam the shield in to the opponent's face.
9. With any weapon equipped, press X for the famous Curb Stomp (Crush the opponents head with your foot).
10. With any weapon equipped that doesn't use the Y button for an execution (e.g. Lancer), press Y to punch the opponent in the face three times til dead.
11. With any weapon equipped that doesn't use the B button for an execution (e.g. Lancer), press B to slam it in to their back. (Note: If you're using the Lancer for this one, just tap B, don't hold it.)

Chainsaw executions and planted Granade executions aren't counted for this achievement.

From Cervantes De Leon | 9th Jun 2009
A quick way to get this achievement is to plant a granade on a downed enemy, then pick them up. When the granade detonates, the enemy will be liquified and you'll be unharmed, adding one to the total. Repeat nine more times till you get the achievement.