I strat I like and works really well.
Get a Gardevoir and teach it Calm Mind, Double Team, Magical Leaf, and Confusion (Psychic at Lv 40)
set up enough double teams to evoid most attacks (heal using potions when needed). After that set up a few Calm Minds and then proceed to sweep. Confusion will be your main attacking move until you get psychic and use magical leaf on his spirit tomb. Don't be afraid to restart a battle if things arn't going your way.