Free the Sorcerer that Darkhunter asks you to take to the Red Tower near the start of the game to unlock three new areas at the end of the game.
Create a knight, and train him to level 40 with a high fire mastery. Fire mastery can be increased by getting some special items, such as Rune Items. Make sure you can hold at least 48 Fire Mana. Capure a Fire Elemental, and research the spell "Fire Siphon". The spell Chivalry is also required. You can learn it at level 40. Select those two spells so that you can cast them. Start a battle, and check the resistance of your enemy. It is best if their resistance is 0%. First, cast Chivalry. It will remove effects such as Poison and will raise you and your enemy's mana to the maximum. In your next turn, cast Fire Siphon. This will drain your enemy's red mana and add it to your life points (not your maximum life points).