What I like to do is run back and forth between Ecruteak and Route (whatever route is below it) and hopefully it'll show up. It's harder in GSC than in HGSS unfortunately. Also, have a Pokemon with Mean Look so it doesn't run away, like Umbreon or Haunter/Gengar. Perhaps posting in the Q&A thread in this section will get you better answers though. That's where this needs to be. Congrats on graduating!