Supposedly, they're weak to water. If that's true, they'll be hit more often with water-type attacks, or even that special of a Pearl (e) bolt.
If you haven't already finished the task, do what Southrend suggested; wear items that boost your prayer, as opposed to your defense. You'll be praying meele, so you won't have to worry at all about being hit.
Since you're planning on meeleing, they
are weak to crush type attacks, but any weapon that uses that probably won't be what you would like to use...Don't show up with a Dragon Mace expecting it to be as effective as a whip. Bring your whip, DDS, a few antifire potions, and your prayer potions, coupled with the rest of the aforementioned equipment, and you'll do just fine
