Kyurem vs The Sword of Justice
Movie Fifteen was released at a time when Dawn was travelling with Ash in the anime, yet she does not appear in the film so some movement is required. Meloetta is also absent, yet during the time Meloetta follows Ash it makes frequent appearances especially when Ash is in peril, so the movie likely occurs outside of this arc.
Making the assumption that the movie occurs before the Meloetta arc, it must
also occur prior to BW081
which sees the group board a boat until
BW082
. This appears to be best place, as geographically there doesn’t seem to
be enough space between Twist Mountain and Icirrus City
for Roshan City (setting of the movie) to exist in.
Source: Bulbapedia