Er@ser
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I advise you not to play with those random swamps and barbarians who charge head-on, but with Castle, Rampart, or Dungeon. If the war has dragged on, then: in Castle, you can recruit "extra" archangels (from conservatories) + archers; in Rampart, there are elves to shoot, along with the most powerful dendroids and insidious (in terms of resistance) "grandfathers"; in Dungeon, there are spawners that are very harmoniously complemented by the Portal of Summoning.
Added after 3 hours and 43 minutes
I want to say that Galthran can, of course, be defeated. I just don't understand how it's possible to play for more than 2.5 months?
And what's wrong with the swamp and the stronghold: I removed the answer and just crush them with a bunch of knights twice, or with a pack of rogues, and there are also archers, and bears can really do some damage, and you also need to build an explosion, and the Necropolis has enough resurrection spells.