>>28554629Mewtwo is strong not only because of its immense psychic power, but also because it was engineered solely for the purpose of combat. When the Pokédex says that "its battle abilities were raised to the ultimate level" it isn't referring to game stats and numbers, but skill in combat.
Things like speed of reaction, the intensity and precision of its attacks, the ability to adapt to the situation and make the best decisions, etc. All of these traits are on point for Mewtwo because that's what it was made to be like. Added to this, Mewtwo has plenty of raw power as well.
Many of the other legendaries are beasts. A Pokémon like Primal Groudon may be able to surpass regular (not mega) Mewtwo in terms of raw power, however it is an animal, and knows nothing of finesse in combat. All it would do is shoot fire and slash wildly in the general direction of a target.
At first glance Mewtwo may not seem like it has a particular gimmick (e.g Palkia having power over space, Kyogre having power over the sea), but that's not entirely true. Mewtwo is a fighting machine. Its "gimmick" is battling.
