Personally, you should wait for the Battle Fronteir to clone your master balls. We all know those make catching pokes FAR easier and quicker
But if you REALLY want to catch him now, then here's a few possible tips. You should try paralyzing it first so it will be much slower and occasionaly won't move, and thus buying you more time. I know putting him to sleep is better with the catch rate, but he wakes up quickly. Then use a medium-powered move or two to get his health fairly low, and then use weak attacks/False Swipe to get his health very low. Then start chunking those Ultra Balls. And if you've taken 15 turns in battle (it might be 10 turns, but I'm not sure) then start using some Timer Balls.
And remember: SAVE BEFORE BATTLING!!