It is indeed true that “mind” can be translated as mens, but also as animus, and the two words are almost, but not quite synonymous and can be a little difficult to tell apart. Reading this belaboured discussion, I suspect the willpower would be better captured by animus, with mens stressing the intellectual capabilities. In your case, I would recommend animus.

(Mens and mentis are the same word, by the way, just different cases – mentis being the genetive.)

Corpus is indeed the straightforward translation for “body.” Yes, it has all kinds of other meanings (just like “body,” come to think of it), but first and foremost it's a living being's body.

The preposition in certainly does not mean “over,” it means “in” or “into” just like in English. Over as in a higher position in a hierarchy would be super or supra, as exemplified by the expression Caesar non supra grammaticos (freely, the emperor has no higher authority than the grammarians). The difference between super and supra was discussed previously on this site; in short, they are both equally appropriate in this case.

So you could say:

Animus supra corpus.

(You may be wondering, what, we can just string words together like that in Latin? Hahaha, no, we normally most certainly cannot, it is pure chance that it works here.)