Set up 3 Calm Minds and use Psystrike on all of his Pokemon, you can only 2 shot Dragonite with Psystrike. WARNING: Use a Full Restore and a Max Elixir before challenging Lance. Set up 4 Calm Minds and use Shadow Ball until her Gengar, use Psystrike on it and her Giratina. WARNING: You MUST use a Full Restore before challenging Agatha, or you will most likely lose. Use 3 Calm Minds then use Psystrike on his Pokemon EXCEPT for his Gallade, for that use Shadow Ball (If his Machamp uses Fire Punch and it burns you, be sure to heal before you move on, as the following Pokemon will likely deal more damage). WARNING: You can use up a Full Restore before fighting Bruno, but it isn't required. Be careful, for her Weavile can use Night Slash which is Super Effective and can K.O the Mewtwo.
Use 2 Calm Minds then use Aura Sphere on her Pokemon (If her Dewgong uses Toxic, wait until her Mamoswine to use a Full Restore, as the Dewgong and Walrein will most likely use more Toxic on you).