so far I have found out this:
1. swordsman -> guardsman
2. priests -> inquisitors (or some of those pansy witch hunters)
3. zombie -> decaying zombie
4. ghosts -> cursed ghosts
5. Orcs -> Orc Trackers (no veteran orcs as far as I could tell)
6. goblins -> goblin shamans
edit:
7. Paladins -> Executioners (demon in Shattera)
I have level 4 available but since I got BK, I just used repair droids and BK until I got to Elon, now using dragons and BK combos, forgot about the diploma long time ago
I couldn't get any undeads to BKs, but I supposse you could train vamps into ancient vamps, but I don't have any vamps in my game that I found so far. Not sure that I need them at this point.
best thing here that I can see for early game is perhaps vamps > ancient vamps (if you can find vamps) OR, I guess if you are warrior you could just go for goblin shamans if you have your anger and rage control built up so you can have level 3 adrenaline. Those pesky goblin shamans are just way owerpowerd (think 100 shamans = 18000LS = 2500 astral dmg per cast, 3 sequental casts in same round = 7500 astral dmg. I had 300 g. shamans obliterate my 3 20.000LS stacks with two casts (I forgot to cast fear on them, I couldn't cast magic lock because they surpassed LS limit on spell)