I agree with a few of the above posters. You don't need to use more powerful, or even different threats. Just have them use different tactics. If the enemy is spread out among the PCs, the area effect spells won't do much good. And at the level you're playing at, precision effect spells, like Mass Suggestion or Horrid Wilting aren't available yet.
Also, the Paladin will be strongest against low AC, high HD monsters like giants (though they're also good against him, they're good against everybody).
Also, the Paladin will be strongest against low AC, high HD monsters like giants (though they're also good against him, they're good against everybody).