Isolating armies is only important in the very early game. After that, you should be snowballing. I'm fighting 3 or 4 armies at once with golems, mages, and 3rd tier swordsmen.
Game needs more content. Bigger map, more obstacles, and more unit types. It doesn't need tactics or anything y'all are talking about, that would change the entire core gameplay loop, which is already amazingly fun.