The 15th mission of COD MW2 'Prison Break' features numerous opportunities to grab this achievement.
While the achievement is self explanatory, there have been issues attempting this achievement before rescuing Alejandro and throughout the level.
I recommend saving the achievement until you enter the cafeteria/break room.
From there, there will be enemies to the back left door, stun them and throw the semtex.
Look to your right to the armored enemy. Stun him (and not yourself as I did in the video) and hit him with the molotov.
The last guy in the way back left corner should be behind a cart. Simply stun/gas him and throw the frag grenade.
The achievement should then pop. If it doesn't, reload from last checkpoint and attempt it again!