Libertine is right: experimentation is everything in this business. I got INCREDIBLY lucky with this girl (which played directly into her becoming my wife). She can get aroused and climax from just tickling. This doesn't happen every time, no more than she gets aroused every time I kiss her or try other touches. But if her mood is right or I find a great spot, the tickling becomes intense, and occasionally when it's good enough she orgasms. Sometimes repeatedly, because I love to keep going.
But she's always multiorgasmic so that's not unique to this. Anyway, it can happen from any tickle spots. Not just her feet.
There is a related point, which may be significant. When my wife is very aroused, touches that would normally tickle suddenly don't anymore and just feel great. So she giggles and laughs, then louder, then stops laughing for pleasure, and then laughs again after she cums. Many women have discussed here a complex interplay between tickling and arousing feelings. Maybe orgasm from tickling alone is a special case of such interplay.
Bottom line: it's not about methods or body parts for her, it somehow just happens when the tickling gets right. But I rarely try for that specifically. Basically, anytime she lets me I tickle, and if all goes well that leads to tickle-sex. So if I'm successful, she'll come one way or the other.