This Pokémon is encountered at the Watchtower Lair's Den when the event item is used.
This encounter is not shiny locked. However, frame advancing via VS or Invite method is presently not possible, as doing so will cause the raid will despawn.
While frame advancing is possible by simply using the crystals, it is impossible to get more than 2 crystals legitimately, as the Crystals cannot be traded.
As such, getting the shiny would be highly unlikely.
The only way (besides by chance) is to restart your game, and use a save editor to check whether you're 2 frames away from shiny. Only then, redeem the serial code and encounter them.
This is the info for this Pokémon:
Note that even though Work Up is the extra move for 1st and 2nd shield, the moves are disabled.