tahirahmed: Ostmurk is optional that I can say for sure because I fought kayran without it.
Actually I did a mistake by using all Ostmurk to create another potion (don't remember the name) but its the one that cause 50% intoxication. I later found that I need that to create a potion that helps me to fight kayran but the quest already entered into next phase (as I found ostmurk in cave earlier and wasted it), Cedric didn't helped then because its either Cedric or cave that you can choose.
So I turned the whole jungle upside down and didn't find any Ostmurk :( therefore I finally decided to face the consequences of my mistake and face Kayran without potion after all this is the fun element of RPG games ;)
Ok during fight I found a nice thing which DJHigher already mentioned that Quen sign is very useful in this fight. It allows exactly one hit to be totally absorbed and thus prevent any damage from that hit, what's even more wonderful is that it allows you to resist Kayran poison effect, you will still stuck in that poison but you won't loose any health.
There are two tricky parts however, first you need some practice for switching between QUEN and YRDEN sign during fight, that can be a bit difficult, the second thing is you vigor won't generate as long as Quen sign is active so for example if you have 3 vigor blocks you will use one for casting Quen and the other two for YRDEN so don't just waste them by casting at odd locations.
QTE takes a bit practice as well but fortunately I died more by tentacles than QTE miss clicks or key presses. I say play a lot of Capcom games for QTE practice :D
Actually you can make that potion without any ostmurk. How do i know? Because i manage to create 2 potions of that.