You get the best physical and magical skills from the dark lord. The first form teaches you Tempest (an electric spell which, with the witch class and every magic improving equipment I could get, was doing above 50 hp to the dragon), and with a defensive strategy it is not too hard to get it and beat him, even if those 999 hp look scary, as long as you keep either shield or the def up food up (I beat him with Warrior). Demea should keep you alive, with a bit of help of defend and a herb here and there.
The second form teaches Raging blow (a 3-5 hit physical attack), and while it has a much lower hp, it can be tough to keep him alive while surviving enough time for him to cum on Feena.
About the elixirs... as far as I know you'll need to replay it again and keep them (I reached the first time the final boss with about 3 elixirs, after using at least 3, if not 5, through the game, so it should be around 6-8 per playthrough), or use the debug menu to get 10.
There is new game + in the game.