For Mewtwo, the basis was Mew. For Shadow, the basis was not, actually, Sonic — at least not in universe. The legendary ...

Z-A remains unknown: the legendary Groudon, Kyogre, and Rayquaza. When Mega Evolution was first introduced in 2013, only one legendary Pokemon was able to Mega Evolve: Mewtwo into Mega Mewtwo X and Y.

The best Nidoking counters are Primal Kyogre, Primal Groudon and Mega Garchomp (as well as Shadow Mewtwo). However, if you don't have these then your strongest Psychic-type, Ground-type ...

Utilize specific counters like Mega Gardevoir, Mewtwo, and Mega Sceptile for the best chance to defeat him and earn Shadow Pokemon rewards. Follow steps to find and rematch Cliff as needed.