|Give It a Whirl|
One method to accomplish this is remove a character's grenade mod and lure an enemy across the top bridge and throw a grenade at them. They should fall off the bridge and land below hopefully in the whirlpool. Furthermore, a singularity grenade offers a strong pulling effect that can drag opponents into the water.
Maya can phaselock an enemy as he jumps across the gap in the bridge, and simply allow the timer to run out, and the enemy will be dropped directly down into the whirlpool, completing the challenge.
Zer0 can execute an enemy as the enemy is on the gap of the bridge, and the execute override skill will knockback the enemy off into the whirlpool, completing the challenge.
Salvador can use a Fistful of Hurt when an enemy is on the gap of the bridge, and the override skill melee attack will knock the enemy into the whirlpool, completing the challenge.