There is a 1% chance that when you activate the herb you will go goblin. This gives you a permanent +50 health boost, +7 rage/turn, and a legendary weapon, "The Green Goblin Glock." You must keep on buying the herbs until they fill up your entire inventory- the risk is worth it.
The 1% is just an estimate- After using the log (which generates a herb) like 5 times every combat, for an estimated 20 combats (5-ish combats per room, 2 rooms per zone, I got the log at the start of zone 2) I went goblin after about 15-ish combats.
I have no screenshot, so you'll have to try this out for yourself. Keep on generating herbs, and eventually you are bound to go goblin. This is the only way I was able to beat the boss.