Use good Pokemon and fight them down in their areas (Veilstone, mt. coronet, the other dimension etc.)
I recomend use of Pokemon/moves that are effective against dark, since this is their most common type.
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you go to sunyshore city and beat the gym leader
well i have beat them in Pokemon diamond and u have to gust cheap on trying to beat them
no. in DP he disbands team galactic when you beat him. in platinum he disappears after you beat him in the distortion world.
In the galactic warehouse to the left of the Galactic HQ
No, instead there is a Team Galactic.