Its always good to have a range of diiferent type Pokemon. If you have any Pokemon that the gym leaders Pokemon are weak to then you should use them.
I used my INFERNAPE,MACHOMP,STARAVIA,SHINX,MIVREUS and my hapiny to beat the fourth gym leader on Pokemon pearl.
you can go there after you beat the gym leader in SUnyseashore. hope this helps:)
haunter
Beat the GYM LEADER [=
you beat her with a fire type
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you beat the gym leader
The seventh gym leader
Find ALL the trainers beat them all, and then fight the Gym Leader at the front.
Beat the gym leader
Beat the league
The Fifth Gym in Pokémon Pearl is the Ghost Gym so use Dark-type moves.