For the first Boss: It's his mechanic. He tries to push the group to the center (later with fireballs), so if there is a possible way to go behind the boss, then this mechanic would be useless. It's better to attack him with less potency from the front or flank then no damage. They could have made him disappear and placed him out of the arena and non attackable. It's ok and it's not last forverer. (20 seconds or so? maybe less).

Second boss: the front is where the eye is. I don't need a ring with a tip on it to figuere it out.

Fourth Boss: The back is, where the cape is. The flank is where the arms are.