Giovanni

Giovanni is the boss of the Team GO Rocket. Grunts and Team GO Rocket Leaders are his subordinates.

Finding Giovanni
Giovanni can be found using Super Rocket Radar that can be acquired only from Team GO Rocket Special Research, after defeating each Team GO Rocket Leader at least once.

Tracking down Giovanni's hideout is not as simple as in case of locating Team GO Rocket Leaders using regular Rocket Radar, as some hideouts marked using Super Rocket Radar may turn out to be PokéStops taken by Decoy Grunts pretending to be Giovanni.

Trainer can also find Giovanni and engage battle with him by tapping the Rocket Balloon that may spawn above the Trainer's avatar in the Map View but only if Super Rocket Radar is equipped.
 * For more information on Rocket Balloons, see: Team GO Rocket.

Battling Giovanni
After locating Giovanni's hideout at last, that is a PokéStop overtaken by him, Trainers can engage battle with him just like in case of Team GO Rocket Leaders.


 * For more information on battling any of Team GO Rocket members, see: Team GO Rocket.
 * For more information on battling any of Team GO Rocket Leaders, see: Team GO Rocket Leaders.

After defeating Giovanni, 5000 Stardust, several Potions, Revives and maybe a Unova Stone will be rewarded. Then Trainers can encounter and catch the Shadow Legendary Pokémon used by him. It has a 100% catch rate.

Winning against Giovanni counts towards the Ultra Hero medal.

Giovanni's party
This is current party of Pokémon to be used by Giovanni in battle with him.
 * Only one of Pokémon in the middle column is being fought in the single battle with Giovanni and this Pokémon is random. Pokémon highlighted with green can be caught after winning against Giovanni.

Changes to Giovanni's party
From the release of Battle against Giovanni, until now, there were some changes to Giovanni's Pokémon party. Each month Giovanni has different set of Pokémon that can be fought.

Trivia

 * Giovanni can be considered a Team GO Rocket counterpart of Professor Willow, as they both are rather special characters in game, and they both have three subordinates/assistants.