Source: Discord

Maybe decouple Skull of Corruption toy from the "Guldan kills" statistic, since it can only drop for Demon Hunters. If you can somehow have it only count kills when you're on a demon hunter, that would be ideal.

Since kill statistics override manual attempts, you can't adjust the count manually in this case. Additionally, limiting attempts to individual classes doesn't work (separate issue), which means any Gul'dan kill increases the counter and not just those achieved on a Demon Hunter.

The workaround here is to use the NPC method, which unfortunately isn't as reliable. All previous attempts will be deleted, but since they're completely inaccurate anyway it probably won't be a huge loss...