This is just the first of a series of achievements that you get from grabbing enemies and throwing them over the edge of a high place instead of trying to punch or use other powers on them.
No matter how you look at it, this will be a grind (though this particular one can be pretty quick, involving only 5 enemies). I've found the best place to rack these up in minimum time is in Mephisto's realm. Initially, you will end up trapped in several small areas where you will need to solve puzzles. One of the puzzle areas is perfect for this. The "lake of fire" area.
Just stand near any ledge, grab enemies as they approach and then toss them over. The best way is to face away from the edge and toss them backwards over your head by pushing the stick in that direction. You can also do this with a side toss. However, if you try to face the ledge and throw them forward they will usually bounce off an invisible barrier, never falling to their death. When you are successful in tossing them over, another will respawn right nearby so rinse and repeat until you are sick of it or have all the achievements related to tossing enemies from on high (Ultimately you need 500 in all).
When you're ready to leave the lake of fire area, just toss 5 of the demons into the lake and the pedestal will come down allowing you to port back out again.
Another opportunity for this is the walkways in the skull cityscape, however, this is counter to getting the "good samaritan" achievement, plus it is not as quick and easy as the first method.