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I would switch my troops to local ones as soon as I arrived at the elves' territory. Two stacks of dryads allow you to clear almost all elven lands without losses. Two pockets are always enough to change tactics. Either archers in a pocket, then bring out melee troops, or melee troops in a pocket, divide the dryads into two stacks, and the archers and fairies take out the sleeping enemies. It's boring, but very effective. And the Ruby of Angir shows what it's capable of. There's also a strong combination - a fairy gambit, time warp on the fairies + mirror of pain. I specifically chose troops only for the battle with Karador. But I didn't return to other islands. If I encountered ancient ents, I would definitely take them; they are the most resistant to undead attacks.
P.S. The Mirror of Pain (I have 108%) - it hits very weakly for some reason.