There are a couple “must do” for certain heroes in formations.

If you use Baal, you want him in front line as he has counter, it doesnt trigger on his guard.

If you use Uruk, you want her to be in the back line, whilst your highest DMG is front line.

Guna and Phoenix want to be in a spot that would most likely have one of their DMG heroes, to kill their sources.

Themis wants to stay alive lol, best spot for her is one of the areas NOT against enemy phoenix, which could even be front spot sometimes.