I realise this topic hasn't been replied to in quite some time, but thought I might post my strategy for taking down the Koschey. I played thorugh this game on hard my first time through, so I've never had to not use potions, and I've gotten quite good at their implementation. I took down the koschey without dying (though for a minute, I thought I was going to), and potions are really your best friend if you want to bring him down. I went through about 20-30 XD.To begin with, you should probably learn the effects of the secondary substances, and apply them accordingly. Also, when you get the chance, drop a gold talent into potion tolerance and a silver into altered metabolism...it's worth it. The potions I went into this fight with were:20x Swallow w/ rubedo20x White Honey20x White Rafford's Decoction w/ albedo20x Tawny Owl w/ albedo20x Willow w/ nigredo(As you can see, I stay stocked to the max on potions =D)I was level 37 when I tackled this monstrosity, but then I had done every quest in the game, neutral path.Also, wield moonblade and use diamond dust on it. Koschey is supposed to be resistant to most effects, so +140% damage is a great replacement for detrimental effects. Before going into this battle, down a swallow, tawny owl, and willow. I used strong style mostly, though I'm not sure if another style would work better. So go in and start swinging. When you're health runs low, blast him with Aard to knock him down and start running away while casting Quen on yourself. Take White Rafford's Decoction, then White Honey, then the other 3 potions you began with. Keep blasting him with the Aard and running when he gets close until you're able to drink all of the potions. Then go in for more sword swinging. Repeat until dead. Hope this helps your future playthroughs. =)