DISCUSSIONS on “Rules for online games in HoMM3 and formulas for calculating player rating.”

In my opinion, it's a rather synthetic example. It's much easier to kill a single unit with a death ripple/arcomage/chain lightning, and the risk of losing angels and other units in case of shackles/Chain Spell/resistance is a sufficient price to pay for the advantage.

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How does the combat ability of a Fairy differ from the attack of an Archdevil or the shot of a Titan?

It differs in that it falls under the rules of hit-and-run.