Which level 7 creature is the best, and why?

I think your preference is justified, but you don't seem to consider Phoenixes at all – in my opinion, they're pretty good guys :) (why? – I don't think it's worth writing about...:))

They have relatively low health. Of course, there's an ability that allows them to be resurrected, which could be considered as compensation, but the tangible benefit from it can only be felt in some battles.

Well, it's not a super victory... Monsters destroy armies that are not only 4 times larger than their own...

That's quite possible.

Can you give an example of such a "long" map?

A German map (I don't remember the exact name, I'm writing approximately) "La dernier Elementalisten" or something like that.