The only known ways to obtain it without cheating are through the English adaption of the fossil conversion glitch with an Attack stat of 253, a box data shift glitch such as the remaining HP glitch (with a remaining HP of 253), or through arbitrary code execution.
Fossil conversion glitch with Attack stat of 253, remaining HP glitch with a remaining HP of 253, arbitrary code execution, Rival LOL glitch (7 as fourth char).
This glitch Pokémon article is part of Project GlitchDex, a Bulbapedia project that aims to write comprehensive articles on glitches in the Pokémon games.