Well the masterball is always a good choice ;) Iwould go with a ball that fits the conditions. If its night you might want to chose a dusk ball, if its in heartgold try to find a Capricorn that makes a pokeball that would be good against a beldum(I don't know what ball each Capricorn makes!), a quick ball might be good if its right in the start of the battle, a timer ball if the battle has been wearing on for a while, OR you could weaken the Pokemon to a very low health point( if you got a scyther use false swipe to ensure 1 HP left) then use a ultra ball or great ball or even a pokeball.
I myself just recomend the last one because its pretty easy to do.
also if you have a poke walker, you could just have gotten winners path and catch it then.
As of Generation Four, and Probably Generation 5, there is only one Beldum. In Ruby, Sapphire, or Emerald, after beating the champion, go to Steven Stone's house on Mossdeep City, and he has left a pokeball and a note, saying that he has given you Beldum to keep. So, you CAN catch Beldum in an Ultra Ball, but there are no wild Beldum TO catch in an Ultra Ball.
Beldum is found In Steves House. A Poke Ball is on his table, get it and you'll have Beldum.
No ball other than the master ball he is so hard to catch ask anybody
You get Beldum for free, from Steven after you finish the Pokemon League, go to Stevens house and the ball will be on the table.
you can catch beldum at route 228 with poke'radar
if you wanna catch beldum it a mass outbreak on route 228
When you beat the Champion go to Steven's house. There will be a poke ball. There will be a Beldum in the poke ball.
you use a pokeball in Pokemon dimand and pearl you can not get beldum only in Pokemon emerald you have to beat the elite four go to wallys house in mousdeep there will be a poke ball click on it and you have a beldum to get beldum in fire red and leaf green trade it in Pokemon ruby and Sapphire can get beldum too what are you talking about,you CAN catch beldum in diamond and pearl.you have to get the national pokedex and talk to your rivals sister in her house and you can catch it in a swarm Unfortunatly, it shares the " hardest to catch Pokemon" award with Skarmory"-thy both have 2.98 percent catch rates. Arceus is easyer to catch.
if you have masterball use it but if you dont have try to catch it at night and use dusk ball
Steven Stone will give you a Beldum in exchange for a Forretress.
a Beldum is a Pokemon that floats. Its was a Iron Ball Pokemon. They communicate by making magnetic sounds.
The best ball to catch Giratina is the Master Ball but the next best and effective options would be the Dusk Ball or the Ultra Ball.