Kostya, what did you do after encountering the Nagas, and where did you position your Gogs in the battle against them?
And what about your and the Nagas' morale?

I played it smart. First, I attacked the Goblins and transformed 3 Harpies and 27 Imps into 4 Demons. I attacked the Naga with 6 Pit Lords, 9 Demons, 15 Gogs, and a few Imps. During the battle, the Gogs also transformed into Demons. The Nagas had incredibly high morale, and mine seemed to be boosted by the whips, and the Demons' morale also improved towards the end (but it didn't really matter anymore). I ended up with 4 Pit Lords and 10 Demons. My tent was destroyed :(

Against the Dendroids, I positioned my units like this: 7 Demons, 2+1+1 Pit Lords, 1+1+1 Demons, to raise new Demons from the single units. The Dendroids often had high morale, but when only 2 stacks remained, morale no longer mattered. After the battle, I had 5 Demons and 3 Pit Lords left. After the battle, the Ballista had about -20 shots left, which means the battle lasted over 40 rounds.

Against the Devils, I positioned my units like this: 1+1 Pit Lords, 2+1+1 Demons, 1 Pit Lord. We block the Pit Lords with Demons in the corner so that the Devils cannot attack them. The Devils kill a Demon, and we resurrect a new one. When we run out of mana, we throw the Pit Lords into the fray. And in the battle between a Devil and a Ballista, the latter turns out to be stronger. After the battle, a Pit Lord and a Demon remain. The rest of the battles were won without losses, due to the presence of the Devil. In general, I could have saved a lot more units, but for that, I would have had to accurately predict the positioning, morale, and breeding, etc... But this is an online game, so I didn't want to take too many risks. :D