Here are some tips to help you.
1. Download VBA (VisualBoyAdvance) and the rom.
2. Transfer your save file and put it in the same place as the rom.
3. Run it on VBA and use a cheatcode from either Gameshark or Codebuster.
4. Get Deoxys through cheat and save again. Transfer back.
5. Start having fun with Deoxys!