Here's a small mod that does that.
It adds to a special counter how much the monster was damaged and every half a second it will damage the monster for 1 damage until the counter is drained.
So if you hit a monster for 10 damage, it will take 10 more damage over the span of 5 seconds.
You will probably have to tweak the mod to your liking, as I really just made it in 5 minutes without much balancing.
I tried to leave comments describing functions in the code.
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