Glacia is the third Elite Four member, specializing in Ice-type Pokémon. Ice-types are weak to Fire, Fighting, Rock, and Steel-type moves. Her team consists of two Glalie, two Froslass, and an Abomasnow. Glalie and Froslass are Ice/Ghost, while Abomasnow is Ice/Grass. A strong Fire-type Pokémon like Blaziken, Arcanine, or Camerupt will be devastating against her entire team. Fighting-types are also excellent, especially against Glalie and Abomasnow. Rock-type moves from Pokémon like Aggron or Golem will also deal super-effective damage. Be aware of Froslass's high Speed and potential for status moves. Abomasnow's Snow Warning ability will set up Hail, damaging non-Ice types. Your Pokémon should be around Level 60-65 for this battle. Prioritize hitting her Pokémon with super-effective attacks.
