Which level 7 creature is the best, and why?

Simply put, if you're going to compare creatures directly, without any tactical maneuvers, then you can't use any tactical tricks. And, by the way, I don't remember the minimum damage of the creatures; try calculating it that way. Most likely, if the Archmage won't wait, he will die in this case as well. And the chance of inflicting minimum damage is small, just like the maximum damage – you need to take the arithmetic mean.