You can't. You have to trade it from quartz or crystal version.
You have to catch a Mantine. The way you catch a Mantine in to catch 2 remoraids and 2 octilley.
You cannot get mantine in firered only emerald and colosseum can.
catch a magikarp,goldeen,remoraid
you have to catch a mantyke and level up with remoraid in your party.
If you play at firered, OF COURSE there is no mantine, because Mantine is at Pokemon leaf green
You'll need to breed Mantine to get it. It is not unobtainable in the wild.
you can catch it on the ice path. p.s. I was doing a global trade and some doofus traded me raikou for mantine. go figure.
Mantine sounds like Manta, and Mantine is a manta-ray shaped Pokemon.
Colosseum can get mantine.
you cant get mantine in soul silver because it is for heart gold only...meaning it is a version exclusive pokemon...so the only way is to trade it from heart gold :D hope i helped...
Mantine are quite uncommon but appear on the sea-route between the Whirl Islands and Cianwood. I would take a few ultra balls and dive balls, as these dont like to be caught