Any character can use the shove bonus action, but high strength and proficiency in athletics will help. Open hand monks and swords bards have special moves to knock a character back. Archers can fire a special thunder arrow that knocks enemies and allies back, too. Also look for chances to destroy a platform that an enemy is standing on.
The problem is that falling only does a small amount of damage and an endless fall won't get this achievement. Work on this with injured enemies in the early game.