Contrary to popular belief, I don't know everything about Pokémon. I'm simply making my recommendations off of what I know. For example, I was under the impression any status effect would affect the catch rate. If Timer balls work better than Ultra or Dusk balls, then by all means, use them.
Given the fact that confusion doesn't change the catch rate, I'd revise my previous comment to simply recommend having Shedinja use False Swipe to bring Beldum's HP down to 1 before commencing with a hailstorm of Pokeballs of your choice. This balances the chances of knocking out Beldum (zero with this strategy) with the chances of it knocking itself out (also zero). You really just need to make sure you have enough Pokeballs.