You have the opportunity to get this achievement very early on in scenario campaign, specifically the "South Bay Warehouse Area", during this level you'll have plenty of people to hit into the water, the best thing is, in this level you actually progress alongside the waters edge!
Try and lure some enemies toward the ledge, these enemies are pretty weak and will die within a couple of successful attacks, so lure them toward you, quickly step to their side and then use "B" to perform a high kick which with a little luck will send them flying into the water below.
As I said there are plenty of enemies during this mission, it also seems to count toward the achievement if your AI partner hits an enemy into the water and if an enemy somehow falls into the water by themselves!
* Credit to RoosterTeeth for the video