Use ninjas: shinobi 1 or 2, stealth when in zombies shooting range, then do mass damage on the zombies with shinob, then run back to the castle and heal (garrison, your ninjas probably have half hp remained and would be poisoned)
Repeat those steps blablabla, clean all zombies, then repeat those steps on juggerknights :P
Use giants: throw stones to the knights and zombie, have one or two medic to cure your poisoned giants, knights and zombie will be no match with your giants
Or you mix with giants and ninjas, I beated that level with insane mode :P