I just got my 8th badge. I got Dialga, Uxie, Mesprit, & Azelf. Has to use my Master Ball on Mesprit. Im gonna go for the Elite Four after I train my team some more.
How hard is it to find Mespirit? Just randomly find him in the grass? And does he run away after awhile? I'd say it's a wise choice to use the master ball on him.