Flannery's team consists of a Level 24 Slugma (Fire/Rock), a Level 24 Numel (Fire/Ground), and a Level 26 Torkoal (Fire). Slugma is slow but has decent defenses, and its Rock typing adds weaknesses to Water, Ground, Fighting, and Steel. Numel's Ground typing makes it immune to Electric attacks but still weak to Water. Torkoal is very bulky and can use Sunny Day to boost its Fire-type attacks and activate its White Smoke ability (prevents stat reduction). The best strategy is to use Water-type moves. A strong Water Gun or Surf from a Swampert or Pelipper will be highly effective. Ground-type moves are also great, especially against Slugma and Numel. Be cautious of Torkoal's Overheat, which is a powerful special attack but lowers its Special Attack after use. Keep your Water or Ground-type Pokémon healthy and you should win easily.
