Curious if people struggled on viper or volbaino more?

Keep seeing a lot of posts about viper but after farming his set he boils down to an erratic first phase and a very telegraphed second phase. In fact if you get past the first phase the second phase should be nothing in time to the point where healing was very optional when farming him.

Volbaino however, rocked me for a good few hours of sheer pain and I would hate to farm him multiple times. The combination of the slowly building dot, the area control and the damage he put out got me so many times. Literally sighed with relief when I beat him and no further boss (so far) has given me the grief that he did.

What boss did you struggle on more?