Pokemon X and Y Walkthrough: Elite Four Siebold
Siebold specializes in Water-type Pokemon, although Clawitzer is his only pure Water-type. His Pokemon are weak to Grass-type moves with the exception of Gyarados. Your Grass-type Pokemon need to be careful of Barbaracle's X-Scissor and Gyarados' Ice Fang. The stats of Siebold's Pokemon vary greatly. Clawitzer has high Special Attack, Gyarados Attack, Starmie Speed and Special Attack, and Barbaracle Attack and Defense. None of them have particularly high Special Defense, so a move like Giga Drain will do very well.
Elite Four Siebold