Use a moon stone on it.
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Jigglypuff will evolve into Wigglytuff with a Moon Stone.
route 3or4
No matter what game it is, you need a moon stone to evolve Jigglypuff.
jigglypuff, clefairy, ridonan (forget if thats the name, i evolved mine, then used the moon stone to turn it into nidoking)
You cannot evolve Murkrow in FireRed.