ok i havent beaten it but you need all pokemon around level 70 (ish) or higher (i think). mine are all at 100. you need at least 2 ghost pokemon and 1 poison pokemon and the rest dont matter. but the ghost and poison pokemon will not be affected by the muk. the other 3 must be pulled out when muk gets near them so be careful.
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go to the next gym leader.
use 3 seakings with aqua ring or water fall and you are done
Soul Badge (Fuchsia Gym)
The fifth Gym can be found in Fuchsia City. The leader is Koga, who specialises with Poison-type Pokémon.