Without buying units

[Gatts]

Well, still you need a lot of luck to get sacrifice... only if you would restart the game only for sake of getting sacrifice in the nearest cities... I actually remember getting sacrifice in hadar first... but I also believe that the mage academy has unlimited random generator, so it could contain sacrifice, if I am right...

So, you can get plants, fire dragonflies, spiders, snakes, skells and vamps... later on its griffins, ents... and even dragons... I could imagine playing like this max on normal... so if you go for impossible, good luck!