Flash
Enchant creature
Enchanted creature gets -2/-0 and loses all abilities. (Mutating onto the creature won't give it new abilities. It can gain abilities in other ways.)
If a creature has power and toughness written as */* with an ability that defines its power and toughness, its base power and toughness become 0/0 when it loses all abilities. Mystic Subdual then makes it a -2/0. If its power and toughness are written as */*+1, it’s -2/1, and so on.
If the affected creature gains an ability after Mystic Subdual becomes attached to it, it will keep that ability.
Mutating the enchanted creature won’t cause it to gain abilities. This is because mutate modifies the creature’s base characteristics, while Mystic Subdual applies to the resulting creature.