Zappo
Explorer
Elementals are very dangerous to a fighter of the same CR, and undead are a rogue's bane. The idea that not all classes are equally good against all foes is rather common. My party spellcasters deal with outsiders by taking Spell Penetration and hoping for the best. This isn't so bad; tanar'ri for example tend to have SR equal to CR + 8, so a wizard of the same level with GSP only needs to roll a 4. Even if you only have SP, and I think that taking SP once is a good investment, you need to roll a 6. That's a problem, but it isn't crippling.