Okay,so first I saw the pokedex and it said that articuno's location was in route 13.So I flew there and checked the pokedex again and saw that Articuno had changed its location to another route! He keeps doing this.How can I actully encounter him?
You are flying to the location
You should go to the location by foot but the bicycle and the rollers can be used. Otherwise, flying to the location will change the location.
Source: Experience and Reddit.
You cant fly to the locations otherwise he will fly off so you have to walk and repeat this 13 times then go to the azure bay on route 12 and find sea spirits den
Just ride your bike back and forth between route 7 and camphrier town. Encounter 2 Pokemons each time you pass through. Keep encountering him till it shows in the pokedex its in a cave.