To add to that. Another option you have, which might be easier on the items (and it's slightly easier than getting an Umbreon imo), is to have a Synchronize lead with Skill Swap. Skill Swap it on Beldum, switch to Misdreavus/Mismagius and Skill Swap Synchronize back. You won't get any damage and Shadow Ball easily OHKOs the Beldums, even if it's a Jolly Misdreavus. Also, Misdreavus gets Thunder Wave via TM too, which is nice for catching.
And just to be sure, I don't know in what ball you want it, but it might be a good idea to pick up an Alolan Marowak with False Swipe too so it actually becomes possible to catch without it fainting to recoil from Take Down. You could go even further beyond and use a Harvest Exeggutor with Skill Swap, Trick/Bestow and a Leppa Berry. If you Skill Swap Harvest on Beldum and Trick a Leppa Berry, there's a really good chance it won't ever faint as long as you keep a Ghost type in.
I've managed to throw 999 Heavy Ball to Beldum this way (only to discover it's impossible to catch later. ;___; ). Do note that if you're doing both Harvest and Synchronize Skill Swap, you'd have to have two Beldums out of course. One for Harvest/Leppa combo, the one that stays alive and the other to get Synchronize on Misdreavus. Just an idea. ;)