To make the most of Pokemon GO's All-Hands Rocket Retreat event, trainers will want to defeat all of Team GO Rocket's Leaders and complete their research tasks.
Finding and defeating Team GO Rocket Leaders will require a little more effort than simply beating up grunts.
Trainers will need one specific item called a Rocket Radar to find any of the three Team GO Rocket Leaders. Coincidentally, however, this item requires players to beat up grunts.
By constructing a Rocket Radar, trainers can find Sierra and her comrades Arlo and Cliff.
Constructing a Rocket Radar in Pokemon GO
Before trainers can find Sierra in Pokemon GO, they'll need to assemble a Rocket Radar. This item comes in two varieties. A standard Rocket Radar is assembled from Mysterious Components, while a Super Rocket Radar is often a research reward used to locate and battle Giovanni, Team GO Rocket's Boss.
In order to receive the necessary Mysterious Components required to construct a Rocket Radar, trainers will need to defeat six Team GO Rocket grunts. Each grunt will provide players with one Mysterious Component. When six are collected, players can construct and equip a Rocket Radar.
With the Rocket Radar equipped, trainers will want to keep an eye out for Pokestops occupied by Team GO Rocket or the team's overhead balloons.
Activating the Rocket Radar will point out a Team GO Rocket Leader's hideout if one is located at nearby Pokestops. However, the radar will also convert occupants of Team GO Rocket balloons from grunts to Team GO Rocket Leaders.
There's no guarantee that the Team GO Rocket Leader encounter will be Sierra specifically. However, if trainers defeat Arlo and/or Cliff, their chances of encountering Sierra should vastly improve.
Considering that All-Hands Rocket Retreat's special research tasks require trainers to defeat all three leaders, it certainly doesn't hurt to beat Sierra's comrades before taking her on.
Regardless, the Rocket Radar can be built multiple times, allowing trainers to battle all three leaders. Sierra isn't guaranteed to appear first, but she'll appear eventually.
Sierra's current team of Shadow Pokemon is made up of different Pokemon types. She begins with the Psychic-type Drowzee but also employs the services of Sharpedo, Lapras, Blaziken, Shiftree, Nidoqueen, and her trusted Houndoom.
Trainers should build their battle teams accordingly to counter any of the Shadow Pokemon that Sierra may utilize. Since the Rocket Leader can only bring three Pokemon to battle at a time, trainers will have to prepare for as many possible options as they can.