Spanish is more "regular" than English is, but it does have it exceptions which have to be individually learned. But I don't mean, memorized. I mean, one needs to become familiar with them through immense exposure .Drilling is the best way for us to get the needed exposure.
It wouldn't be wrong, but the difference is subtle in this case, because there's presumably no doubt that she's a person. «Ella es una persona fuerte.» means that, unlike some other people, she's strong. «Ella es una fuerte persona.» means that she's a person, and, in the speaker's opinion, she's strong.