It helps to use combos and attacks that launch enemies far. Launching enemies into the air by holding
and then hitting them to push them while they're in the air may also work..
As XiRe x Harvey added:
Towards the start of the first mission, when you walk over the ledge and the helicopter flies passed you, run over to the helipad and wait for the enemies to follow you. All you do is press B on an enemy when they're near the edge, then press A and push LS towards the edge and you'll throw them.
This is a much easier way to knock an enemy off the edge. It's essentially just grabbing and throwing the enemy.