Hehe~ ♥
It's easy to get the EXP required for the desired nature, but for Shininess it's quite a bit different as you'll soon find out~
According to a few sources, Pokemon obtained in Red, Blue, and Yellow can only become Shiny when traded over to Gold, Silver, and Crystal with specific DVs. As far as I know, said Pokemon must be met under specific circumstances (i.e. starter, gift, interactable (so Mewtwo counts), etc.), and obtaining said DVs is about as random as IVs in future generations (in addition to the Shiny rate in GSC being the usual 1/8192). Ultimately, it comes down to luck whether you actually get those DVs... but certain other unspeakable methods exist that can guarantee those DVs upon transfer without the need to trade to GSC, which is how a fair few people here have obtained their VC RBY Shiny Pokemon.
So, in short, getting the Shiny is really difficult when putting in an honest effort without the use of those unspeakable methods. I hope this answer suffices~ ♥ (and should I have misspoken or left anything out, feel free to correct me)