My understanding of Wellington's way of fighting was that he was very capable strategically (i.e. he acted to keep his army in a fighting condition in as many ways as possible), operationally (he only engaged when he felt he had a good chance of winning) and tactically (his repeated victories against pretty much every French general against whom he gave battle more or less show this).
Basically, you need him to screw up badly in at least one way. (And when he came to Spain, he was already experienced in India, so he's not going to make stupid mistakes.) Probably the best way to have him defeated is to have him misapprehend the nature of an enemy force, leading him to give battle incorrectly.
Assuming that, how do we get that to happen? One possibility is that a French general observes his counter-tactic for the Column and comes up with a counter to the counter. Spinning ideas here, but what about if he has a radical departure of a tactic which involves the column basically passing loaded muskets forward for the men in the front to fire? It's kind of tricky, but if you can have two columns launched at the British line at once, one of them a conventional one, and have it be screened by this "firepower column", then the departure from normal French tactics might - might - allow for the second column to punch through Wellington's line.
That's an idea, and it might well not work. There may be other ways of substantially and unexpectedly altering the French tactics, as well.
I'd say a good time period to do that in is either in the very first battle in Spain (when he makes a category error - treating French troops like Mysorean ones) or about halfway through (when he may have fallen into a pattern). It's a tricky one, though, precisely because of how good a general he was...