I've also found Nascour's Metagross much harder to snag than Evice's Tyranitar so I end up snagging Metagross with it....
Yes, I agree that the Metagross was hard to catch... But the Master Ball was meant for the Ultimate Pokemon, which is the Tyranitar that Evice has. And I don't think you can get another one...
I think I used that one on Metagross on the second playthrough.
On the first playthrough I saved it and sent it to my Sapphire version.
Go Penguins! (Piplup evolution line and hockey team included)
I use it on everything once I have it, thanks to the duplication glitch :)