Inferno Belial was the most challenging boss I've had to fight so far. Other than that, I had trouble against Normal Diablo, Inferno Ghom, and...that's it. I've died once or twice against some of the others, but that was usually getting one-shotted carelessly or something.
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All the stars in the sky are waiting for you.
Diablo 3: DarkMac#1527.