I submitted that enemies going after downed creatures is not what DMs do in D&D and takes away from the fun. Getting them back up is already a resource drain and it is spell slots if theyre surrounded, which are finite. Definitely bad behavior.
Them focusing the weakest person, every time - also annoying. Threat generation needs to exist, though clerics and wizards are good things to make sure you target. Shield and mage armor can start making that a bad idea. He gets a power that lets him reflect projectiles - arrows, bottles, etc...pretty OP when its a bunch of jerks doing that haha. It shouldnt be necessary, though and blowing a second level mirror image every fight because they do that isn't enjoyable.