Mercenary Retention Question

[Darkstone]

Somewhat tangental, but mercs die off pretty quickly. I realize they're supposed to cannon fodder that conveniently appears in the right place at the right time, but I suppose I'd like a few tougher and more expensive merc companies for the mid and late game. Something on the level of gargoyles, wights, ether warriors, frost fiends, banefire archers, lead by something tough/magical/skilled.