"Give It a Whirl" challenge, and "knocking" enemies

The "Give It a Whirl" Bloodshot Stronghold challenge states:

Kill an enemy by knocking him into the whirlpool.

My melee only slashes—how can I knock an enemy?

This challenge is fairly difficult, especially for the Gunzerker and Commando, whose special skills aren't of much help. edit: it was pointed out in the comments that I misread one of the Gunzerker skill descriptions. I haven't tested it, but it appears that he has an ability that works much like the assassin's.

Zer0 has the execute ability in his bloodshed tree, which can cause knock-back into the whirlpool. Maya's phaselock can be used on enemies jumping over the pool (you can lure them into this by jumping across yourself), which will drop them when it wears off.

Everyone can get this achievement using grenades, but it's tricky. The best grenades for this appear to be vanilla -- they don't do too much damage to kill the bandits outright and they're explosive, so they blast people away from them. MIRVs don't appear to work too well, despite their explosiveness. The phaselock-like grenades would be better if you can get them to go off above the pool.

Actually this is very easy to get simply by using Singularity Grenades. Throw one into the pool and all enemies, even those very far away, are drawn towards it. I've gotten people from the top of the ledge to fall down into the pool this way. This has a bonus of not damaging enemies at all, so you can retry any time you want (grenade count permitting) and you won't kill the target unless they fall in the pool.

I Hope This Helps! This is by far the best and easier way to do it, especially if you are zero since all he does is slash without having any knock-back. This way you do not need to get any skills only a grenade mod. Simply equip any "Singularity" grenade mod, throw the grenade into the whirlpool once enemies are somewhat close to the edge and voila! You get the achievement. In the video the achievement was previously awarded, hence no achievement awarded during video but I assure you it works!