zabuza
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I don't really like fighting against archers, but again, it depends on the situation.
If we're talking about the most annoying units, here are a few:
Vampires - especially in the early game. You can defeat them, but their regeneration can be very frustrating.
Poisonous creatures - when there are many of them, the poison is dangerous.
Djinn - strong and have magic.
Efreet - strong and deal fire damage.
Medusas - their petrification is a real problem!
Well, I think that's all, the rest are manageable.