In my opinion, this kind of tactic goes right into the box labeled "Things that we would expect in real life but in which require us to suspend belief to make gaming fun."
This kind of tactic is the same as putting in the old "puzzle" to get through. For example, in another thread the example of a wizard's lab containing a secret passageway is described. Since I don't know the thread, I'll make another example up:
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The party enteres the wizards main study room of his library. They know that the wizard keeps his two most significant spellbooks in a secret room. Inside the room is four walls of floor to ceiling full bookshelves, a chair and desk, a miniature crystal ball on a gold stand resting upon the desk, and a brass candlabra. The miniature crystal ball appears to have one hemisphere filled with a gray cloud, and the cloudy side is pointed in the direction of the chair. What they don't know and have to figure out is how to activate the secret opening.
Here's how it is done, and three things must be accomplished. One the third shelf of the bookshelf behind the desk is a book entitled "Magical Trapfinding." That book must be pushed back so that it is against the back of the bookshelf. This will produce a single click. Second, the miniature crystal ball sitting upon his desk must be rotated 180 degrees so that the clear side is facing the chair. This will produce a second click. Third, the brass candalabra must have all the candles taken off of it. This will produce a third click and the secret door opens.
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My point? The above scenario makes for a good entry in a novel or a movie. Because it is scripted. It makes for a poor game because it halts the PCs ability entirely (unless you allow for an INT check to figure out the puzzle - bt at that rate why go to the effort of designing a cool puzzle to allow a dice roll to beat it? Just don't include it!). Furthermore, it is precisely the thing I
would expect to find in the wizard's inner studty! So, we must suspend reality and for the sake of "fun" ignore that this is precisely the type of thing that we would actually find.
The same is true for your counterspelling agent. That is precisely the tactic that we would expect to find in a fight, especially if the BBEGs know it and are high enough level to do it. It makes great sense! But, we must suspend reality and claim that this type of tactic is off limits because it interferes with fun.
That is the nature of our game. It is unfortunate, but true. This would make a great battle in the novel because the author is in control of everything and writing about the characters is the fun. It makes for poor tabletop experiences because the fun is being stomped on by the counterspeller.
Just my 2 cents, though.