The Lesser Demon Champion attacks with both magic and melee. It uses very powerful spells, but has rather weak melee attacks. The Champion uses only magic from a distance, and will use both melee and magic up close.
During the fight players cannot wear any armour or weapons. Therefore it is almost impossible to sustain the damage of the Lesser Demon Champion. The only way to kill him is punching him to death. Dreadnipscan be used, contrary to popular belief. A good strategy to do this faster and saver is to drink super pots or overload and use Turmoil and Protect from magic. In addition to that one must use Momentum ability before entering the chamber. Don't forget prayer potions and some food as well.
Due to an update you now need a weapon to cast spells you can't use magic against him anymore. Also it is no longer possible to bring Batwing and Thread in to the arena to make a batwand.
Since the Lesser Demon Champion cannot detect protection prayers, it is advised to stand back and use the Protect from magic prayer against him. He will not use his melee attacks if the player is not next to him, and will only focus on attacking the player with magic. With the protection prayer on, his magic attack will deal less damage, so bring enough Prayer potions (1-2, depending on the player's Prayer level) to keep the prayer on and enough runes so that they don't run out mid-fight. It is wise to prepare for over 250 spells when going for a sure kill.