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1. Usually, Imps. They have more predictable damage and don't waste a turn on mana drain.
2. Almost always, creatures that can jump; at least they have some use.
3. Always fire-based; after all, hitting ranged units is effective.
4. At first, I took Demonesses, but then I tried Temptresses and started taking them all the time (except against Necropolis).
5. Both are good, so I take them differently depending on the situation (against Necropolis, I only take the older upgrade).
6. Most often, Broodlings. A melee attacker is better than a spellcaster in this case.
7. Most often, I don't use them at all, or I use so few that they are practically insignificant. If I do use them, I usually take the older upgrade.