Well hopefully you just wanna masterball it ;)
But to find it, the only "easy" way to do it is:
1) get a pokemon that is 1 level below the level of your target (latias, what is that? L35? so L34.)
2) get a bunch of repels
3) Go to ANY place where two routes meet and there is tallgrass on both routes
4) open up your map in your pokegear, just so when you open the pokegear it pops up to your map
5) simply walk a few steps until the message pops up saying you entered the new route, then check your map and see if Latias is in your route
6) simply walk a few steps back until the message pops up saying you went back into the route you came from, then check your map and see if Latias is in your route
7) Repeat, and within just a few minutes it WILL randomly end up in your route, just be patient
8) Use the repel and walk around in the nearest tallgrass patch in that route. Because your mon is 1 level below Latias, all the wild mons won't show up, only Latias will
But to find it, the only "easy" way to do it is:
1) get a pokemon that is 1 level below the level of your target (latias, what is that? L35? so L34.)
2) get a bunch of repels
3) Go to ANY place where two routes meet and there is tallgrass on both routes
4) open up your map in your pokegear, just so when you open the pokegear it pops up to your map
5) simply walk a few steps until the message pops up saying you entered the new route, then check your map and see if Latias is in your route
6) simply walk a few steps back until the message pops up saying you went back into the route you came from, then check your map and see if Latias is in your route
7) Repeat, and within just a few minutes it WILL randomly end up in your route, just be patient
8) Use the repel and walk around in the nearest tallgrass patch in that route. Because your mon is 1 level below Latias, all the wild mons won't show up, only Latias will