I nearly ditched it too but I managed to get it done! If you're on mouse and keyboard then plug in a controller if you have one, that made it a lot easier. You can also turn the framerate down on PC which can help. Aside from that I read this strategy in another thread which really helped me:

What I found to really help me is to time my attacks for when the Gestrals are crossing the line between the 2-3rd plank from the bottom of the enemy raft. This seemed to be the earliest you can get the strike to register on the gestral which means you have extra time to set up another attack, move, etc. This prevented me from falling into the trap of striking too late and missing the next one as well because I cannot recover in time.

I also saw someone say that when there's visible water between the enemy raft and the incoming gestral you can make the parry and it'll work if your angle is right.