If you want, you can just give me the .sav file of the ROM with the Shiny Swellow and I can transfer it to my White and trade it to you through Wifi.
What you need:
1. An unpatched GBA rom.
2. An unpatched DS rom (preferably D/P)
3. A blank DS cart (R4, Supercard DS, etc)
What to do:
1. Save your GBA game in a Pokemon Center. Yes, this does matter.
2. Open Desmume and load your DS rom. Go to Emulation > GBA Slot > Guitar Grip and load. It will reset.
3. After resetting, go to Emulation > GBA Slot > GBA Game and select your game. It will reset.
4. Load your ROM, you'll see the Migrate option and just do it.
5. If you get a "Save failed" message while migrating, try re-downloading Desumume and the DS ROM.
6. After the transfer is complete, do NOT close Desmume. You have to go to File > Export Backup Memory and save it as a .sav file.
7. Put the ROM and the .sav file on your blank DS cart and load it then trade it to another D/P/Pt/HG/SS game, then transfer it to Black.
There are tons of videos of this on Youtube, and many written tutorials like this. If you don't have a blank DS cart, you can get Wifi to work through Desmume so you can just trade it from your computer to your DS but I've never been able to get it working. Hard. D: