1st form-33 hp, 2nd form-49 hp, 3rd form-65 hp, 4th form-79 hp. Before going to fight him, talk to the relekka helmet seller and talk about koscheis sword, then go and kill koschei on the 1st, 2nd, 3rd, AND 4th forms, he will always drop his fremminik blade on the 4th form ONLY if you talk to the helmet salesman. To kill the 4th form, use several rings of recoil as he only hits 1\'s, but 3-4 hits per second.
Possible strategy: use 6 Rings of recoil (these shatter) and high healing food lobbies or better (editor\'s note: at combat level 75, 14 Oomlie wraps weren\'t enough). Do not use Protect from Melee prayer - you must take damage for this to work. If you take any damage at all wearing a Ring of recoil it reflects back 1 damage. In his fourth and dreaded form Koschei only hits for 1 damage albeit very quickly he kills himself by hitting you and having the damage sent back against him. 43 prayer does not work here because it prevents all damage.