ArvelDreth speaks true. Sebastian is highly sensitive to the steam but not to Kain kicks. Once on the platform above the ground, come close to Sebastian , press the R1 and green triangle button to grab Sebastian by the neck, then press the pink square button to throw him down in the steam. It is useless to try to kill him in regular combat.
It's funny what happens in the cutscene if you defeat sebastian using immolate, also you need invincibility too because kain can still be killed in the cutscene.
Well done ! I have come in this fight countless times and so far I have still not defeated Sebastian. I have noticed a few tricks. At the beginning, do not move Kain while Sebastian is burnt twice by the steam. Then he starts to move around the room. To avoid the unblockable red kick, I jump if I am fast enough. If you please have any further suggestions ?
I know I am late to this. When Sebastian has a glowing red move, side step (auto focus + left or right arrow+ jump). You can jump but most likely it is too slow. Side step can change the game for you.
In order from hardest to easiest boss fights In order of most annoying to least annoying fights 1. Sebastian 1. Magnus 2. Marcus 2. Faustus 3. Magnus 3. Marcus 4. The Sarafan Lord 4. The Sarafan Lord 5. Faustus 5. Sebastian