I used this trick, it works very well. A fun fact to know is that repels only repel lower level pokemon. go to a route with weak, low leveled pokemon. Bring a bunch of max repels with you. then go near grass where there is a building to the next route. Then use the repel, with a LV 39 pokemon leading your party. This will repel everything but the legendary. Give it a few steps (about 20) if you don't get anything go to the building (technically meaning you go to a different route, thus, changing the roaming legendaries location) and go back to the same grass as before. repeat this until it happens to appear in the route you're in. Be ready because both will try to flee, and if trapped, they will use roar to end the battle. this repel me about 10-15 minutes to find a roaming pokemon like entei or raikou.