The Fak'ough Demon is actually the first demon you end up fighting in the game. It's also your first death. It initiates the story line, and actually just means fuck off demon.
Fak'ough Demon has 500 HP and 200 mana, so prepare to die on your first encounter. On your second encounter in the Good Campaign, he has the same stats, albeit easier now that you have already increased skills like Weapon Attack and Weapon Defense (or Magic Attack and Defense if you're a mage and Archery if you are an archer)
Note: this is gameplay advice that worked on some players, and may not be accurate to your gameplay. This particular one is on an avatar that has 200 HP and 180 Mana.
Ascended Campaign Edit
The second time you encounter this enemy, you have the stats and the spells to kill him (ASSUMING you went and cleared the Spells Campaign). Use the blessed weapon you obtained from the angel with the armor she gave earlier to prep yourself. Great Heal and Weapon Enchant is a great help , so stock up on Major Mana Potions or Rejuvenation Potions if you want to focus on fighting. Weapons with Mana Burn, Burn, and Bleed can also help whittle down his HP. Alternatively, you can use Artifacts stronger than the blessed weapon.