To easily gain HP, create a knight and train it to level 40 with a high fire mastery. To increase Fire Mastery acquire special items, such as Rune Items. Make sure you can hold at least 48 Fire Mana. Capture 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 like Poison and will raise your 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.