Go to cerulean Cave, Save in front of mewtwo, then engage him. If hes not shiny,(Which is indicated by a different color) Soft reset (which is start Select L and R for gba) and rince and repeat until you get one*
* Note there is 1 in 8921 chance of getting a shiney, so this may take a long time
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By cheating.
1/8192 is the appearance rate for any shiny pokemon. The odds are no different with legendary pokemon.
Nature has no factor of whether or not you find a shiny Pokemon the nature will change depending on the Pokemon of course now if your catching a shiny mewthwo then everytime you catch a shiny mewtwo it should have different natures granted it's possible to have one of the mewtwo's to get that same nature of the first one or the others.
In Victory Road
if can go to cerulean cave you can