I am completly sure is a KO with max att, Scizor with it and CB deals 84-97% to Reuniclus with the much weaker U-turn.Another counter is Escavalier, if Trick Room is set up. Barring a Esca not running a 0 IV (honestly who the hell is it going to outspeed with a 31 speed IV), it will outspeed most in Trick Room and while I do not have the calculations, I would venture to say that LO Megahorn OHKO's. Esca is also pretty damn bulky on both sides that even Focus Miss shouldn't do much.
And I shouldn't have to tell you that Psyshock being double resisted is not going to jack and $hit and jack left town.
But the problem of Escavalier is, it is not easy to fit in any team.
Reuniclus is not much a threat if you know its moveset, Team Preview helps this.
Like past posts, Deoxys-D is a great counter to CM Reuniclus, with Taunt, Night Shade/Seismic Toss and Recover.
Mew is too with the same moveset(Roost/Softboiled for Recover, obviously or even Super Fang instead of Night Shade/Seismic Toss)
Latias can win CM against it unless Reuniclus has Psyshock.
Gardevoir and Alakazam can Encore it.
Really any Psychic can deal against it as long they keep it to store boosts.
And of course, Wobbuffet is a underrated counter to it. Really it is the perfect counter unless it is the last pokemon. Wobbuffet is faster too.
TR is easy to deal against, because it is not going to boost its stats, and its physical defense(and special defense, but less) is not too good without any investment. Just stall the TR turns and hit it hard.