The probability of getting the Unicorn Horn and Amulet in Castle Keep

The way I did it seems pretty fool-proof. I just ran in and popped an invulnerability potion right before I touched the boss. Then when it runs out teleport back. When you get to the boss next time, the timer still won't be up though, so you'll have to use another teleport scroll, but then the next time you can turn invincible again, and repeat the strategy until victory. It's pretty costly, but the winnings will pay for it easily.

I suggest using only major healing potions for the first round, while directing yourself to the parts where the demon clusters are the thinnest. Use a major healing potion as soon as the cooldown is over, especially at the wall. What you want to aim for are single demons. Don't be afraid to hit those, since you have more than enough HP to survive.

Once you get teleported back, go to the bottom row as soon as possible and summon an imp. It will kill the ghosts as well as eat through a wall. On your final teleport back, go to the bottom row again (avoid fireballs whenever necessary) and summon another imp, which will cut through the hoard of ghosts to the demon. Use invincibility and spam fire scrolls, and you should be done.