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1) Buy a shit ton of max repels
2) Get a Level 50 Parasect with Spore (level-up move) and False Swipe (egg move)
3) Go to Route 24/25, in the grass area above the ledge
4) Use your repel with Parasect leading your party
5) Run around the grass for a bit
6) If you don't get an encounter after a few seconds, move into the next route, then go back into the grass
7) Repeat steps 5/6 until you find it
You don't need to breed the Parasect but it makes it much easier to catch if you do have one. I'd just use the Masterball if you have it.
