i dont know if im missing something but this guy is casting his spells way too quick and often. too many times when hes about to use gangway, i run to the platform to survive, only to have a .3 second window to run off before his other spell hits me and knocks me off to my death. Now if im lucky to survive the first "gangway" cast and make it off the platform he usually cast his knockback spell 3-4 more times than he either casts "gangway" instantly or quick enough i dont even see the casting bar pop up also without any text warning like the first time and get instantly killed. i hope this is a bug and not intentional as i dont understand the point of making these SOLO zones so frustrating and damn near impossible for the solo player.
I totally agree it took many deaths until what I call pure luck to beat him. The first couple of deaths before I understood what I needed to do were acceptable but after that experiencing the same issues as the OP was frustrating & not fun. I don't even want to take my alts or my main into that encounter.
If you're running to the platform for gangway, you're doing it backwards. At least in my understanding of the way you've described it here, it sounds like you're doing it backwards. For gangway you want to step off the main platform and onto the little side docks.
In solos, depending on your DPS, you may be able to just ignore Gangway. If he hits you with an incurable detriment, that's what you run onto the fishing platforms to cure, then immediately back off them.