Let's not forget that resurrection does not work on level 5 units. Paladin's prayer skill doesn't work on them either.
The only two things that come to mind and can be used to revive level 5 units are the demonologist's drain ability (transfers life to any unit) and the inquisitor's resurrection ability. There is a way to have them to that resurrection a few times by using the phantom spell - the amount of creatures you get from it can be improved by the summoning skill.
You should also note that level 5 creatures aren't exactly the best. My perfect army consists of Paladins, Knights, Archmagi, Demonologists and Gorguanas or Inquisitors. In this army everyone can help each other, everyone can resurrect each other and there are plenty of units to phantom to good effect.
__________________
|