The current monster mentality is basically "attack casters and ignore melee fighters as much as possible". Doesn't matter if you are with real people or heroes/henchmen. I can recall many times where there are two (or more) melee characters in a monster's face but who does the monster continue shooting at and use spells on? The non-melee characters, especially monks. All the casters could be out of its aggro range but the moment one crosses that barrier, monster almost immediately shifts all focus on that caster and stays there. Happened to me lots of times.
Enough already! Many others will agree with me that this crap needs to end right now.