You may want a Repel and to have already caught a Bagon. This makes it easier because you won't encounter any other Pokemon (like the annoying Golbat) and you don't have to wait for a Bagon to pop up randomly, because it's registered in the DexNav so you can just search for it.
So I would stay near the cave entrance (to the room that has Bagon) and when you search for it, if it keeps saying "It couldn't be found nearby. Try looking in a different spot!", exit the cave and reenter and try searching again.
I hope this helps.