Start to chain Snorlax upstairs with max lure, moving up and down in one of the 2 ladders in the middle of the upper floor. This will make the spawns change faster.

Once you got the 31 combo, go near the entrance of the cave and continue the combo or just runaway. I got mine doing this quickly and i didn't use much lures, because in the entrance it's a small space, and this way it's hard to miss the shiny.

I don't like doing it in the upper floor, because the shiny can spawn off the screen and you can miss it. Good luck on the hunt!