I would divide level 7 upgraded creatures into two groups: those with a speed of 11 or higher and those with a speed of 10 or lower.
And then, depending on the map: the availability of your own magic and the magic used by opponents, the troops you'll have to fight, etc.
For example, if I managed to get a fire book, I would prefer black or gold dragons; with an earth book, archangels look good; with a fourth level of magic obtained from Pandora — there is a wide choice (especially depending on the main hero's skills — water: archdevils; fire: black or gold dragons; earth: archangels or archdevils). As for phoenixes, in sufficient numbers they are good with almost any spells...
It must be admitted that in my experience I usually had to choose between no more than two creatures: in my own and a captured AI castle, so the choice was almost always not that difficult, given the available resources...