you need to get it first before trading it away. the chance of you getting a shiny cressilia with charm is roughly 1/20k
and this is with charm. in reality it is not possible to hunt them with charm cuz once they’re registered in the dex, they can no longer randomly spawn. so actually the chances of you encountering as shiny cressilia are 1/64k, which is absolutely mad. your best bet is pokepon, which is incredibly slow.