This is my expirience when i was helping a friend upgrade his vid card cooling on his radeon with original epoxy.
Freezed it for 15 minutes in a ziplock freezer bag, took a small screwdriver and worked it around the heatsink, dont be too cautious or you wont get it off, then i put a fullsized screwdriver in the inlet i made with the small one and twisted it until it started to give, then i did that all around it, and made one final (fairly forceful) twist and the sink popped off with a most satisfying crack.
Before this i was going very easy on it and it wasnt getting me anywhere. Basically, you gotta' use some force to get it off, while still being gentle enough not to damage the core. Take your time with it and it will work itself off eventually.