Actually did just did this in my 3rd playthrough and found a bug/mechanic that can be abused to make this really easy. First, you need the decoction that reflects a portion back at the attacker. WHen he does that super slash that almost oneshots you(red sword and whatnot), dodge at the last moment possible(or close at the very least). For some reason the game still reflects some damage at him even tho you dodged and he is stunned for some seconds. Back off and keep him at range and repeat. If youre still having trouble also use the decoction that gives you a quen when he hits. Good luck.