Well, in the final battle, there's no point in hiding troops of levels 1-6 in reserve anymore, but of course, you need to conserve resources in the early battles. It's been 4 days since I defeated Biara, but I still can't bring myself to fight Kha-Beleth. There are no doubts that I will defeat him, it's just that I haven't been in the mood to play for several days. I'll try tomorrow, it's the weekend after all.
Good luck and have a positive experience! I meant the first stage of the battle with the Overlord, when you destroy his fire wall with a catapult. Hellfire Wave completely wipes out low-level creatures; you won't have time to resurrect them all.