There were not many dissimilarities between an area boss and a world boss; however, there was one glaring difference.
World bosses, as their name implies, are free to roam the world as they see fit. On the other hand, an area boss was forever bound to their area and could not leave its confines. The area size varied depending on where the area boss was located, but regardless, it was an absolute rule.
Ultimately, this meant that as long as Izroth and the 9th Division could escape the area boss' area of influence, they would no longer have to worry about the Monsoon Death Shades. Therefore, Menerva had the right idea about keeping the division on the move.
As for Izroth, he long since guessed that the creature behind the Monsoon Death Shades was one of intelligence. And, now that he discovered that the creature in question was an area boss, it made perfect sense.
(Unedited)