Tornadus, Reshiram = trade (Landorus = optional)
Cobalion = teach a pokemon Flash to make this search easier, the first 2 floors need Flash, the third floor, The Trial Chamber, is where Cobalion is. ONLY then can you get Terrakion and Virizion.
Virizion = in Pinwheel forest, you go to the place with dark grass, go through there until you see a place that leads into the forest, in other words the Rumination Field, that's where Virizion is.
Terrakion = in Victory Road, you get to the top, using Fly would be the best way, at the top go right, go to Cheren's left, use Strength on the rock and go to the far left, then up, and that's where Terrakion is.
Landorus = after getting Thundurus and Tornadus go to Abundant Shrine with both of them, and have a pokemon with Waterfall, go up and down waterfalls and see a dark spot that leads into the forest, go to a weird bird house thing, there's Landorus.