I took the time to assess a lot of mobs when I was redesigning the frame images, this is my conclusion and results.
Weak =
solo single or double down arrows
Average =
solo no arrows
Heroic =
1 group ^ or ^^
Heroic+Tier = *Not implemented in game or for target window yet.* This might be a placeholder.
Epic =
multiple groups ^^^
(see Raid indication for # of groups needed)
Raid2 =
2 groups + Epic indication
Raid3 =
3 groups + Epic indication
Raid4 =
4 groups + Epic indication
This info was applied to
Moab's Target Window.