Qwerty
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In my opinion, speed is one of the most important parameters (otherwise, Sir Mullich wouldn't be one of the strongest heroes). Without considering speed, I don't see any point in even trying to derive a formula.
Abilities are also important: for example, upgraded fairies work wonders thanks to the combination of flight and retaliatory strike (as well as speed). By the way, the AI ranks devils among the top-tier non-upgraded level 7 units precisely because of the retaliatory strike.
I don't even need to mention vampire lords.