[Endoperez]

If you can spare Water gems, Bottles of Living Water. 10 Water gems each, less if your Caelum has Icecrafters (forge bonus), even less if you have Dwarven Hammers. Summons a new Water Elemental at the beginning of battle, so no losses unless you lose and it doesn't even slow your army down.

There is, however, problem with the 75% hp-loss rule. The elemental has several forms, but only the first one is calculated for the starting hp. That means that you still have to have other bodyguards. Something with high hp, like Wyverns, might help a little.