This one is a bit complicated but it works :P
Get a NidoranFemale in XY, and White (its available on those games)
Get a male Onix in ORAS, XY, BW2, or BW and level it up to lvl 40 in oras, it should learn iron tail.
Breed the onix and nidoranFemale.
After that you should get a NidoranFemale with Iron Tail (Breed until you got the NidoranMale (NOT FEMALE) or you can just breed the female nidoran with the iron tail with a Male Nidoran, and you should get an Iron tail NidoranMale.
After you got the Male Nidoran With iron tail, breed it with a female snivy and Evolve it to a Serperior
hope it helped :D