Condemned 2: Bloodshot

You will need to do 25 environmental finishing moves for this one.

A tutorial early in the game will show you how to do them.

When fighting enemies, once you knock them around enough they will drop to their knees. At this point hit both RT and LT at the same time to grab the enemy. The screen will then show white skulls, these are the EPA areas. Simply drag the enemy to one of the skulls and the animation will activate.

If you don't get this during campaign, you can also get it in the Practice Arena of the Bloodshot Fight Club

It is stated above perfectly by 'ShimmeringCube' on how to do this (beat an enemy to the point that they drop to their knees, the game will then prompt you to pull both triggers when you are close behind them. Then simply lead them to a white skull icon to perform an ''Environmental Finishing Move")
This can be done throughout the campaign or even in the "Bloodshot Fight Club", But I have found an easy place to get these very quickly.

During the first mission you will enter a "TV shop", you will eventually be prompted to do the "Chain Attack" tutorial (this is also the perfect place to complete the"Chain Gang" achievement as this enemy is invincible to 2nd & 3rd level chain attacks, if unlocked).
Using 2nd or 3rd level chain attacks you can beat this enemy into submission prompting the ''Environmental Finishing Move''. Since he can't die from it you can repeat it 3 times, with all the skull icons in the room. (Until you use a 1st level chain attack this enemy is invincible)
You can then hit start->reload last checkpoint (loads you right back in the same room)...and rinse and repeat till it pops.
Even though he doesn't technically die from it, the game counts it towards the achievement.
Hope this helps !!!

Bloodshot Fight Club
Thugs
One at a time
Firearms
Debris

Enemies will come at you in a set order from three different spawn locations. Pick up an assault rifle and aim for their heads (don't lean on the trigger) until you charge up to two power bars.

Once you've got two bars charged, drop the Assault Rifle (remember where you put it!), press RT/LT twice to initiate the 2x Power Combo where you break their arm. Once the sequence is complete, punce the thug once, and they'll be ready for you to grab hold of. Take them to the white skulls dotted around the map then rush to pick up the Assault Rifle again. The Environment Kill should take you up to one bar automatically, so charging up to 2 bars will be quick. Repeat 25 times and the achievement should be yours in ~15 mins.