Interesting.
Magma's fire form makes it difficult for spiderman to really hit her or anything, and when she's in contact with the earth she has a really good healing factor. Then she has projectiles and can manipulate the environment, too. Spiderman will have to use his surroundings well to get wins here, but seeing as his opponent can crumble buildings/mountains/cliffs with ease, he won't have very many places to flop around.