From all the uniques you can randomly encouter, I find the reward you get for defeating Guug quite underwhelming. By the time I normally fight him, I don't care for a ruby stone or two that might drop from him.
I would suggest to change him so that he has a random amulet in his inventory. Thus, defeating him would give you nice reward - generally an amulet of ESP, which is nice, but rarely you could find the other amulets, which are even nicer.
Given we're talking about a reward for an already non-guaranteed unique, I don't see a problem with guaranteeing an amulet drop from him. You won't see him in many games, and you still have to beat him to claim the amulet.
Plus sometimes, he will generate with an amulet of life saving and use it himself instead of letting you take it from his corpse.
(At least I hope monsters can equip amulets in their inventories...)
What do you think?