It should be noted that it is an heuristic, and a cheap and dangerous one. There is no guarantee it approximates the optimal course of action, because it doesn't account for action utility.
In the example, if Mr Armor is a minor threat (e.g. he needs a free hand to fiddle with his phased force-field device and therefore he's armed with a little knife) Mr Rifle should shoot (like everyone else) the most dangerous enemies instead. After successively switching to generic enemies of decreasing priority, Mr Rifle should start shooting Mr Armor when it becomes more useful than contributing to killing another enemy faster. Meanwhile, the rest of the team switches from the last enemy they could attack to protecting and supporting Mr Rifle, since it's the most useful action they can perform.
On the other hand, if Mr Armor has a very high priority (e.g. he throws phased force-field grenades) everyone should target him; of course Mr Rifle figures out he can do it, while the rest of the team, knowing that they cannot attack Mr Armor, can either protect and support Mr Rifle if it's useful enough or attack generic targets; no need for a premature heuristic decision.