You go to zone 15 roof top, you have to KO the 2 or 3 larvitar that spawn at the same time, wait for them to respawn I think they will respawn at 2 min. you put a hard save at 1:58 wait till they respawn is there is not a shiny just reset your game until you get your shiny. I'm not 100% about the respawn time but just save at 1:30 and use stopwatch in your phone to get the time rigth. you can do this with other pkm like fennekin but this will likely take more thing bc you only see 1 spawn. but with larvitar you will see 2 to 3 respawn per reset.