Question of PoR item drops

I’m trying to finish the Great Sage quest in portrait of ruin and I’m only missing Thor’s bellow. Problem is that horseface isn’t dropping one even though my glasses say it should be 10% chance. I’ve been grinding this for over an hour by soft resetting to the save point and warping to horseys floor of the Nest of Evil.

So my question is when is the drop actually calculated. Could I just rewind the last hit and save myself some time. Or do I need to fight something else to reset the drop chance.