Denominus: Managed to kill him on Insane with a Swordsman/Alchemy build. I was actually going to skip this fight since I knew from my previous playthoughs that there was a GOOD chance I was going to die and loose my saves, but decided to give it a try having done the fight twice before.
Basically only used Aard and dodge rolls, I'm not a big fan of Quen because of the vigor penalty. With the higher end weapons and buffed critical effects I one shotted most of the gargoyles which helped a lot.
I didn't have a chance to fight with him yet (after 7 plays, just because I always went and talked to Deathmold first -.-; ) but I must admit that Swordsman/Alchemy build kicks butts.
While doing my playthroughs (not counting first try which was on normal not hard) I went pure sword, pure mage, pure alchemy... then sword+magic, magic+alchemy and sword+alchemy and the last one is just OP when you're buffed up. Not to mention adrenaline bar which was almost always full thanks to passive generation from potions and slaying opponents.
I guess the hardest part for me with this build was with dragon at the end. Not because of dying but it took my 3 reloads to figure out that I can't kill her with bombs while standing near Sile teleport and getting
healed with Dragon fire breath. (Just if anyone wondered you need to lower dragon hp to ~30-50% then hit at least once with sword to get cut scene where you're allowed to go on top of the tower... then bomb her until her hp is at ~10-20% and hit with sword again to get another cut scene, pain in the butt)