Spreadsheet of Useful Equips with Minor Location Spoilers

[Vulture]

Well, you can't really go endless anyway because after Round 20 your mana reg stops entirely.
As an alternative, if you don't have a no-retal unit left you can always
a) hypnotize them (with a Demoness at best, it's on a 2-round cooldown), cast magic spring on them, put poison cloud over them
b) get Ancient Vampires in humanoid form and do the same, placing them next to that unit. The Ancient Vampire will regenerate after each turn and not lose a single unit.
c) get Vampires in Bat form and do the same only leaving the final blow to the Vampire so they can drain life up. Just make sure it's no plant or undead ^^

About the defense: why is it surprising ? You need insane amounts of defense to make a noticeable difference at such high costs (atk, int, ldr instead?) that it never pays off.