Good question, and it doesn't look easy. One method is to teach the TM to a compatible male Pokemon in Generation 4, migrate to Gen 5 and breed with a female Snivy.
If you aren't able to do that, then you'll need to chain breed. One method is:
- Get a male Aron that knows Iron Tail via level up (level 39).
- Breed with a female Mareep/Flaafy/Ampharos to get a male knowing Iron Tail.
- Breed that with a female Snivy.
You may also be able to do one of these:
- Aron -> Nidoran(m) -> Snivy
- Aron -> Rhyhorn -> Snivy