Make S.S. Anne Stay
Make sure you catch a L3 PkMn in Pewter City (but not Jiglypuff or Wiglypuff).
At S.S. Anne beat everyone even Gary but leave the people in the rooms before Gary (don't even go there).
Then, Withdraw the L3 PkMn at leave the others you have (deposit them) via PC. Get CUT from the Captain then battle someone and you'll lose and black out but S.S. Anne will still be there.
Now you can do the Mew cheat.
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