Huh. I didn't know Beldum only had Take Down and Struggle. OK, I've done some research. Both are normal type moves, so simply use a ghost type as your attacker. I'd recommend Shedinja because it learns False Swipe as its pre-evolution. It won't be able to put Beldum to sleep, but it can use Confuse Ray, Will-o-Wisp, and Toxic. I'd recommend Confuse Ray for the lowest possible chances of knocking Beldum out. As stated above, I'd recommend doing this at night, when you can make full use of Dusk Balls. If you don't want to risk an accidental KO, simply have Shedinja use False Swipe and lob as many Pokeballs as you want. Beldum's Take Down will fail on Shedinja, preventing it from damaging itself.