Oh hey it's this guy. Yeah idk, probably Unova. Very few native fairies and tons of poison and steel-types to combat them. The only native fairy-type only shows up around Pinwheel Forest, and the non-native ones are only around both starting towns and the Giant Chasm. In terms of your other criteria, the best fit for "Fairy-type Pokémon on your team is the worst idea ever", "many Steel-type, Fire-type, and Poison-type Pokémon" and "the Pokémon on the cover are resistant and effective against Fairy-type Pokémon", your only option is Johto. Even though it has quite a few fairy-types, pretty much all of them suck and there are tons of anti-fairy Pokémon throughout the region to scare them off.