Lati_s will be threatening for sure. But without Soul Dew, the only thing Lati_s has on Mewtwo is really Levitate (which is good, certainly). Mewtwo packs a stronger punch and is faster.
And Ice Beam will be (or at least should be) ubiquitous, what with Ray, Groudon, Grass-types, and Lati_s running around. I don't think there will many things packing Shadow Ball to be honest.