So, I understand that wyverns are needed to cast poison on the ifrits. Could you use spectral dragons instead with the same effect?
I have one tactic against him, but I think it's too weak. Take phoenixes and harpies. Cast antimagic on the harpies and surround the titan with birds, but I highly doubt this tactic will work. Because you'll have to resurrect the phoenix every turn so that the titan doesn't shoot. And the last battle with titans and rust monsters made me realize that a harpy without blessing will deal 5-25 damage to titans, and there are 500 of them.
The tactic with phoenixes and harpies won't work. There won't be enough mana to resurrect the phoenixes and enough speed for the harpies, unless the terrain is very specific. And you'll also need someone to constantly destroy the cloned titans.