The problem is a matter of numbers. If every enemy is trying to shove me off a cliff, they shouldn't also be able to do damage.

Regardless of that though, your last sentence seems to be implying that the player should have just not positioned themselves that way, but I regret to inform you that there are a lot of fights where you don't control everyone on your side. I save-scummed a fight three times because my ally spent his first turn every single time running straight into combat and standing on a peninsula surrounded on three sides by lava.