The most recent releases in the Pokémon franchise, Pokémon Brilliant Diamond and Pokémon Shining Pearl, have added a requested feature to the series' iconic gym battles: the ability to rematch Gym Leaders. After the Pokémon league has been completed, players can rematch each Gym Leader, fighting them once per day.
As would be expected, these fights are much more difficult than their story equivalents. Not only are the fights higher leveled, but the news teams are EV and IV trained, and feature the use of competitive items, providing a challenge for more seasoned veterans of the series.
Candice
Candice, a Gym Leader who specializes in Ice-types, sports a vastly improved team over her debut one. Her team revolves around a lead Pokémon, Abomasnow. Abomasnow's ability, Snow Warning, activates hail, dealing damage to all non-Ice type Pokémon, as well as augmenting other abilities and moves. For example two of Candice's other Pokémon have the ability Snow Cloak, which raises evasiveness in hail, while her Glaceon has Ice Body, which allows it to heal while under said weather.
While her team can hit hard, especially from powerful attackers such as Weavile, all Ice-types suffer from a bevy of easily exploitable type weaknesses in Brilliant Diamond & Shining Pearl, which most high-level teams will have an answer for.
Volkner
Volkner's team focuses on Electric-type Pokémon, but interestingly, also contains a Pelipper, a Water and a Flying-type. However, Pelipper's ability Drizzle activates the weather hazard rain, which allows his electric-type core team to use the powerful move Thunder with perfect accuracy, making him a difficult Gym Leader and a massive threat to teams with many electric weaknesses.
A number of Volkner's Pokémon also have a focus on high speed, such as Luxray, Raichu with the item choice Scarf, and a Volteon with the ability Quick Feet, which boosts its speed when suffering from a status effect. While this team can pose a major threat, five of Volkner's Pokémon share a weakness to Ground-type attacks, and Ground-type Pokémon will also be immune to Volkner's thunder based onslaught.
Roark
Roark is the rock-type Gym leader, and like several other gym leader rematches, uses weather to great effect. By using a Tyranitar with the ability Sand Stream, Roark can set up a sandstorm for his team, activating abilities such as Sand Force and causing damage to all Pokémon without a rock, steel, or ground typing. His team also features highly defensive Pokémon, such as a Probopass.
While the high damage output of Roark's team can be a major threat, some of his Pokémon, such as Armaldo, have abilities that do not activate in a sandstorm. Furthermore, Rock-type Pokémon have a massive number of weaknesses, including common types such as Grass, Fighting, and Ground.
Gardenia
Gardenia, a Gym Leader focused on grass-type Pokémon, also uses weather as a primary gimmick in her rematch. In her case, she uses sunlight to heal Pokémon with the ability chlorophyll, as well as use the devastating move Solar Beam, without its usual one-turn charge time.
Gardenia also has a team that is well balanced between special and physical attackers, which can easily threaten a player's team if it is too skewed. However, several of Gardenia's Pokémon, notably Sunflora, have low base stats, compounded by grass types many weaknesses, which can be exploited by shrewd players.
Byron
Byron, whose team focuses on Steel-types, uses entry hazards to threaten opposing teams, as well as a team of his own that focuses heavily on physical damage over special damage. Notably, his Aggron possesses the ability Rock Head, allowing it to defend itself from the recoil its move Head Smash would usually deal to itself.
While Byron can easily decimate unwary players, his entire tram shares a weakness to Fighting-type moves, which can easily be exploited.
Fantina
Fantina is the premier Ghost-type trainer in the Sinnoh region, and her 2nd team focuses heavily on status effects and debilitating opponents. From using Destiny Bond to knock out the Pokémon of unsuspecting players to using the move Trick to switch a negative item to a player's possession and more, Fantina's playstyle focuses heavily on underhanded, unorthodox tactics. As such, players might not have the tools needed to easily counter Fantina, though her team does thankfully share a weakness to Ghost-type moves.
Crasher Wake
Crasher Wake, the Water-type Gym Leader, also focuses on weather effects, specifically rain, in his team composition. Crash Wake's Politoed can use its ability Drizzle to cause it to rain, which will dramatically boost the speed of four other Pokémon in his team, all of whom have the ability Swift Swim, such as Ludicolo (one of Shining Pearl's best exclusives).
To top it off, Crasher Wake's Gyrados has the ability to intimidate, which will lower the attack of Pokémon facing it. While this team can easily decimate players, it is easily stopped in its tracks by activating another type of weather. This will stop Swift Swim from working, making his team much easier to outspeed and defeat.
Maylene
Maylene, the Fighting-type gym leader, does not use weather or odd strategies like many of her fellow Gym Leaders. Instead, her fighting Pokémon have moves and items tailored to their strengths. For example, her Heracross holds a Flame Orb, which activates its ability Guts, giving it a massive spike in damage.
On the other hand, her Breloom has the ability Technician, which increases the power of its weaker moves, many of which allow it to move first every turn. With a number of strategies that need to be specifically countered, Maylene can be a tough fight for unprepared players.