@zjazd18 is right about the Paladin Recruiter skill - I've been using it for almost 200 fights and have been using it to keep my ArchDemon stacks maxed out.
I have 183 ArchDemons in reserve with two stacks of 20 and 19. I've probably recruited 193 of them.
I think, though, they changed the recruiter percent from 6 to 3% of your total Leadership; My ArchDemon Leadership is 1170 now (due to bonuses) and I have about 44k Leadership and 3% of that is 1320. It seemed like you needed to have enough Leadership for at least 1 troop for it to work so it is better used with level 4 and below troops. Since it was 6% before, I was able to start recruiting ArchDemons around 25k Leadership or a touch higher.
So you should be able to max out your Dark Paladin (or whoever else you'd like) stack, once you get it, pretty easily. I don't think it works with Dragons, though, and certainly not with Dragon Riders (I think these troops are not available in any normal shops, only special ones) as I have never gotten any of those troops from it.
/C\/C\
|