This will be possible starting tomorrow. Niantic has announced that Shiny Rayquaza will appear in the five-star Raids from Wednesday, July 31 until September 2.
In Continental Portugal time, Shiny Rayquaza will start appearing from 05:00 on 31 July. After this, you will have until the same time on September 2 to capture the coveted Shiny Rayquaza.
Rayquaza is a Dragon and Flying Pokémon, so it is weak against Ice, Dragon, Rock and Fairy attacks. Of these attacks, the best ones are really Ice’s, as Rayquaza has a weakness quadrupling against them.
To improve your chance, we advise you to read our Rayquaza Go – Raid Pokemon Guide, Counters, How to Capture.
Good luck on Pokémon Go Raids!