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1. Whenever you deem appropriate. It’s best to do it towards the end of the battle to avoid further losses from this unit, but ideally, at least a quarter of the unit should remain – there’s no point in raising corpses if there’s too little left.
2. Weak units. For example, you destroy the gate. You push two units with 1 health into the opening, and the enemy is delayed for one round. Although, I don’t personally use this tactic.
3. Read the description of the ghosts. They have a passive ability called “spirit.” As long as they don’t move and do nothing, they take 50% less damage and can fight back. But, if you want, ghosts can also be used as a healer or an attacking unit. The problem with the above-mentioned tactic is that the ghosts must remain in their starting position at the beginning of the battle, or you have to use teleport on them.
4. Well, well. The game is interesting, but you don’t want to spend money? You want to figure it out, but you want to delete it? Decide which option you prefer.