i looked at some showcases it's seem serpent's fury has better dps but has to stack frenzies b4 cast right ?
Yes correct, I struggled with it at first but have gotten the hang of it now with the charges. It's very easy once you get going on how it works. Can easily enter to boss with charges already at max from killing mobs and one shot the boss.
If not, cast snipers mark one or two times (more if Xesh or Arbiter at higher tier) and then cast serpent, poof boss dead