Go upstairs and earn an elemental spear using 2 primed mind bars. Go back to the body space, wield your brand new spear, and pull on the lever at the center. While fighting, your stats will be boosted dramatically. You may wield only mind armor against the Homunculus. After the fight is finished, your brain armor will have turned into Elemental Balance Armor. This armor has gentle melee stats. The spear (or, if you have improved sufficient through barbarian training, hasta) has medium melee stats, also can auto cast spells too.
Go Back to the Ancient Cavern and try to go through this Mithril Door. You will succeed. At the center, the creatures will vanish, and also a phantasm appears. He clarifies that the Nightmare Creatures are pawns of whomsoever summons them, so the two Zamorakians and Zarosians can utilize them. The Phantasm has been revealed to be part of Zaros's soul. He'll strike you, inflicting huge harm if you don't have at least one bit of elemental balance armor, or even the weapon.
He uses Ancient Magics, including Miasmic, as well as ranged and a melee attack with an energy whip. Defeating him is more of a battle of wits than many battles you fight. He appears in the middle of the point, does an emote, disappears, starts an assault (in the ground, not at you, so you can dodge), disappears, repeat and rinse. I will not go into specifics about what happens, come up with it yourself. If you know what attack is coming, but not where it is going to be you can use astrology, however everytime you use prayer to block an attack, then your prayer is drained several points.