Can't believe I am even suggesting Madden "strategy" tips but let me add this disclaimer:
Although inspired in part by watching real NFL games, the following tactic is fictional and is not an attempt to depict any actual NFL tactic or strategy.
That said, although I have tried and continue to sometimes experiment with multiple counters against slants and other things, I find the best method to be entrapment, whenever possible. One of the biggest weaknesses of people with very little tactical diversity, is their predictability. So like others have eluded to, completely adjusting your playcalling to account for one tactic, while sometimes very effective against that tactic, can leave you open to other things and probably cause your opponent to attempt to try something else. Now, them trying something else might not sound like a big deal but I find it advantageous, although not very fun, to have a general idea of what my opponent is going to do versus not.
So, if defense A counters slants well but leaves me wide open for runs up the middle, screen passes and post corner routes versus defense B that counters screen passes, post corner routes and offers reasonable defense against middle runs but leaves the slants wide open, I opt for defense B. The reason being, I am not only in a better position to defend more, I also have the advantage of knowing what my opponent is likely to do and can manually make a calculated adjustment. Slants remind me of blocking shots in basketball or defending the goal in hockey, no matter what, all shots are aimed at relatively the same place. So the slant will likely be thrown to either the left or the right side of the field, decide the side you want to manually cover and use your defensive playcalling/player coverage adjustments, to account for the other. Besides, it's a good rule of thumb in Madden not to get enamored with one defense to stop one tactic because that is often not consistently the case anyway.
I think one of the biggest mistakes people make when trying to be successful at Madden, is trying to directly apply real NFL strategy. I have found very few tactics in Madden to make sense in relation to similar NFL tactics and sometimes they even seem to work the opposite of one another. Without getting into a list of obvious reasons, the main one pertaining to defending slants is the gameplay view, imo. Do to being able to see so much of the field when dropping back to make passes, attempt runs and defend, there is very little necessity for wholesale adjustments to playcalling and doing so will often cause far more harm than good, in Madden.
Honestly, just try to think of defending repeated slants in Madden as facing a "boss monster" in an arcade game and try to figure out their attack pattern, so you can pick a good spot to minimize your damage while attacking. IE, Space Invaders, lol.