What is the area icon?
The image on the left IS the area icon. The image on the bottom tells you two things, what arcs are affected by the area attack, and which arc(s) are currently weak to attacks (granting an extra black die). Anyone in an affected arc within range will be hit by the attack... Not so bad if it's range 0, but if it's range 2  it'll hit practically the whole board. Also remember that anyone on the line between two arcs counts as being in both... so someone to the North East of the boss would be in both the front arc (and get hit) and in the right arc (and do extra damage).