Fume knight is bloody hard. Did you do the thing that stops him regenerating?... Can't remember what exactly as it's a while since I did it.
Fume Knight was the biggest boss challenge I've faced in any Souls game yet.
Part of it was just my build/strategy, though. I'm running 2 handed great/ultragreat swords, basically no armor, no shields and I've also only put like 4 points into my HP.
I figured out his first phase *relatively* quickly. Still took me about an hour before I could get him to his 2nd phase more often than not, though. I could eat one hit, but that was it, and I had to make sure I healed right after cuz I definitely couldn't take a second.
The real trouble came with that 2nd phase. Was such a pain for several reasons. Main one is that I couldn't get hit once, by any attack. Was instant death. My other problem was that he completely changes everything - his moves and his timing, which tripped me up time after time. He could also recover super quickly from a bunch of his attacks. Took me forever before I could even last like 30 seconds in the ring with him, since one hit and it was all over. This made it really hard to learn his moves and his timings.
All in all, I probably spent about 8 hours over three days trying to beat him. I refused to summon help and wanted the satisfaction. And boy, was it satisfying. So far, he is my favorite boss battle in Souls along with Artorias, who was easier to learn with less moves, but was still an epic and fun fight.
Haven't done Alonne Knight yet, though. Heard he's pretty awesome, too.
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