Shadow and Mewtwo have similarly traumatic backstories ... and their tears magically transform Ash back to flesh and blood.

Leader Cliff is back in Pokemon Go with a new lineup for April 2025, including Shadow Cubone, Machoke, Annihilape, Marowak, ...

Primal Groudon and Mega Garchomp (as well as Shadow Mewtwo). However, if you don't have these then your strongest Psychic-type, Ground-type, or Water-type will do as it's weak to these.