...try to capitalize on either the chaos of battle, separating the group, or catching them while resting/sleeping.

Also, if you're in a position to scan and see them without being noticed, or are in the same room and somehow are hidden from them, you wait for them to over-extend via a skill or spell (preferably with 2 or more rounds of lag tacked on to it), and then strike. If you can incapacitate via sleep or blackjack or something, don't hesitate to put that on someone who's already asleep as the group recooperates...they might not even notice it and it will make the whole attack easier.

Another thing to do is to figure out who the group leader is and then hit one of the _other_ group members while they are being led around. If the leader is spamming directions or isn't paying attention the fight could be over before they even notice.