Armored Mewtwo

Mewtwo's power-dampening armor significantly holds it back. Armored Mewtwo's Attack is equal to Girafarig, Poliwrath, and Sandslash. It will likely see as much use as any of those Pokemon despite its impressive bulk.

Dynamic Punch is an upgrade over traditional Mewtwo's Focus Blast, though you'll definitely want to stick to Fighting-types for that job.

Unless there's a way to unlock Mewtwo's full potential in the future, the best use of Armored Mewtwo is to hold onto it for its PvP utility.

Best PvE Offensive Moveset

Confusion + Psystrike is Armored Mewtwo's best STAB moveset.

Dynamic Punch is also a good move and can be useful for gym clearing. Stick with Confusion for higher DPS and EPS.

Confusion is not the terrible Iron Tail, and is thus the fast move of choice for Armored Mewtwo in PvP. This is because it is good and not bad. Psystrike is an incredible move. Incredible damage, incredibly energy efficient, with really solid neutral coverage. While it's not a strict upgrade over Rock Slide, the additional damage does it pretty huge favors in neutral scenarios.

Rock Slide is preferred for its coverage and relatively lower 45 energy cost, which also makes it the ideal choice to bait shields. Dynamic Punch is an excellent Fighting-type move that costs 50 energy and also covers Mewtwo's weakness to Dark-type opponents. Earthquake and Future Sight both cost a steep 65 energy, but the former provides wide coverage while the latter takes advantage of STAB. Generally speaking, the Psychic-type damage from Mewtwo's fast moves makes Future Sight somewhat redundant.