You can also use brooches for additional difficulty and rewards with teammates. The stages also get their own unique modifiers once you do them on higher difficulty, changing up the way the enemies play. So it turns into a situation where you and your teammate have to manage the mobs well and plan things out. This means that after you did your stuff, the enemy could attack you (with buffed stats), then your teammate acts, and then the mob can attack you again, without you having any counter play. The reason is because the enemy team has 1 unit that turns in an elite, increasing its stats and making it act in both your and your teammates turn.
Co-op is definitely challenging, often even more difficult than solo depending on your setup.
