There's a mod for the Poketch that has a map on it for tracking him down. It's the one that has a map on it with the symbols at the bottom. You get it for free in Jubilife City from the Poketch Company after you get 3 badges. When you leave one area and enter a new area, he randomly moves to another area, so you need to be in the same zone as he is to catch him.
What I've been doing is going to Jubilife City and go out to the south to Route 202 and watch his position on the Poketch. Then just keep going north back into the town, then back south again outside so he'll change positions...do this over and over and eventually he'll be in Route 202 the same time as you are. Then you just walk into the grass and you'll encounter him.
The problem isn't finding him, it's actually catching him. Whenever you do damage to him or try to throw a ball at him, he'll flee imediatly after. I've done this several times and I have his energy in the red, but whenever I throw a ball he escapes from it and takes off again. I don't have any Master Balls <sigh>. So I've been trying to level a Pokemon with the Block ability to be able to do this move to him so he can't escape, but the only Pokemon I have with that ability is a level 44 Bronzong, and he's way too slow to do that move (even if he's holding a Quick Claw) he's just too slow. I'll go to do the Block move, and Mesprit just flees before I get the move off. So I guess I need a Master Ball, but I used mine already to get my Palkia. Sucks.
