Firstly I parralyzed him.Then I weakend him, and then I cought him with an Ultra Ball.
I kept my MasterBall for the Latios,it was really anoying to ran left-right all the time to make him move closer!!!
The very first time I played Emerald, I used a master ball. That is, at the time, I didn't think it was possible to catch legends with regular balls. The second time, I used a timer ball. Third time, master ball once more, because I gave up.
But the fourth time, I used an ultra ball. What luck!
I've started a new data on Emerald once more. Hopefully this time, I'll use a regular ball without resorting to the master ball!