[Xrystal]

Yes, unfortunately, what you are asking is likely to be impossible within wow's allowed functionality. Addons can make suggestions and tell you what to do but only based on what information is available. So unit wise is raidx, player, partyx, target, pet, focus, mouseover and their equivalent targets. So your addon would have to assume your player is at lease monitoring the potential attackers in the area ( I know I do ) and would then allow them to either simply mouseover them one by one or target them, and on the indirect scenario of being in a group of some sort or having a pet then you can do similar tests that way.