Lorelei:
Instead of using fire Pokemon, try electric and grass Pokemon. However, be careful using grass since her ice Pokemon will have an advantage.

Bruno:
Use Water and Grass Pokemon such as Squirtle and Bulbasaur when fighting Bruno.

Agatha:
Three of her Pokemon are ghosts, but all are poison. Ground and psychic Pokemon work best against them. Even though the manual states ghosts have an advantage against psychic Pokemon, it does not seem to matter when fighter her ghosts.

Lance:
Use any Pokemon that knows the ice type moves. All of his Pokemon are weak against that form of attack.

Gary:
Try using Fire, Ice, Poison, Electric, Or Ghost Pokemon types to burn, freeze, paralyze (sometimes), or put his Pokemon to sleep. Also, most attacks do not affect ghost Pokemon.