Well, ML pretty much covered it on the trio.
Strategy for Cynthia: Unfortunately, Spiritomb has no weakness, so just pummel it with attacks. Roserade is a real pain if you don't have a decent Fire-type. For Gastrodon, pretty much your only hope is to call in the Grass-types. With Lucario, pull out that Fire-type again. Milotic is extremely irksome, but if you hit with high-power Grass- or Elctric-type attacks, you should be good. And for Garchomp, bring out Dialga and
it doesn't get hit by a Ground-type attack. So here's what I suggest:
Dialga, Infernape, Roserade, Magnezone, Gastrodon, and Honchkrow. Have all of these at Lv 63 or higher. Hope this helped.