Just had the longest and in the end most successful and satisfying fight I can remember in all my KB playing. I had just ticked over to level 40 but Elenhel didn't challenge me straight away (he must have a chance to challenge after every subsequent fight?). So I fought Demenion's castle for the stone and THEN Elenhel attacked. I was starting the fight with about 20 mana

but I really didn't want to have to reload and replay the Demenion fight, so I gave it a try.
I had heard that Elenhel loves to spam level 3 Geyser but at first he seemed to be using Call of Nature instead, which is a terrible choice for 40 mana and gives something like 150 Red Snakes or something. But Elenhel did start using Geyser after a while and I took a great deal of damage from his units. The Inquisitors especially took a hammering and at one stage I was down to 11 of them! I really should have used Target more.
Anyway, I just about managed to keep the Inquisitors alive, and I was down to about 20 Horsemen and maybe 40 Knights. I could win the fight, but with no losses?
So I left one Necromancer alive and started the arduous task of trying to regenerate health/mana/rage. I started off by Phantoming level 1 Inquisitors (15 mana) as I planned on regenerating my mana/rage that way (I have 25 for 25 Mana Accelerator). After a while I realised that this wasn't going to work, at least not for hundreds of turns. I was about to give up when I thought about Magic Spring again. But how could I make it effective without using Target?
Then I had a revelation. The 1 Necromancer does an area attack for maybe 5 damage per hex, so if I clustered my units together and cast Magic Spring on the middle stack, the Necromancer would always hit it! By trial and error I discovered that level 2 Magic Spring was the most effective - costing 7 mana, it generates 15 mana over three turns. So I was gaining 8 mana every 3 turns, which meant that over time I could generate enough to Phantom Paladin my troops back to full health.
Took 111 turns in total. Screenshots attached.