Killing diamond elementals

[unmerged(719922)]

Hi, after a long time not playing I decided to start back up. My game was going very well after I found that trolls are really powerful. I then went into an elemental plane and after poking around and setting up a few cities I found a diamond elemental camping a seat/throne of the elements (cant really remember which and I don't want to risk going near him again after I looked at his resists.) He is immune to everything except life magic, spirit magic and melee, and even then they have really high resists. I'm not that far into the game and I don't have anything that can deal enough damage to him before his 30 attack just kills everything. Is there any tactics I can use against him?
Thanks.

EDIT: Also I found red dragon eggs which I can get as long as the diamond elemental does not move from where he is right now.

 

[Amorphium]

I got lucky in my last game and got a vampire hero, if levelled up he can get a vampirism aura. Noble Werewolves (30% spirit magic), healer heroes or the unit you get from building a temple to agrela also deal spirit damage. Black Minotaurs with Vampiric Weapon enchant also work if supported.