Mewtwo counters - Ttar or other counters

Personally I'd go with 3-4 Tyranitar and the rest of the lineup resistant to fighting. Even if Mewtwo does have Focus Blast, a Tyranitar should still get a decent amount of damage done, and the other 2-3 Pokemon can survive long enough to finish the raid, and if the Mewtwo has any other charge move, then 3-4 Tyranitars is probably all you'll need anyway.

I think people are forgetting that raids are about damage, and Tyranitar still does a lot of it even if it does faint quickly to Focus Blast.

People don't bat an eye when someone brings a Gengar that dies in 20 seconds and only gets one Shadow Ball off, but somehow think that Tyranitar is a bad choice because it gets one shot by Focus Blast.

Also, based on sims Scizor lasts twice as long as gengar and if the fast move is confusion gengar will need to be above Lv32 (exact level depends on Def breakpoint) to be able to get one shadow ball off.

If you look at this SR post, you can see a non-dodging Tyranitar will get 1 shotted (dodging is hard in raids due to lag and the dodge bug) by Focus Blast as does a lot of other mons...(Sneasel dies 3 times over!)

https://www.reddit.com/r/TheSilphRoad/comments/6vkqqm/understanding_the_importance_of_dodging_against_a/

The 131% is a Level 30 Tyranitar would lose 131% of it's HP meaning that even a level 39 (HP doesn't go up that much more from 30->39) Tyranitar would get 1 shotted.

That being the case, you can see that if you had a lot of Tyranitars, with a lot of trainers beating on the Mewtwo (charging it's energy), Tyranitars would be falling left and right (as is everyone else)...

Gengar against Psycho Cut/Focus Blast would be pretty awesome if you can get a few Shadow Balls in, but you will most definitely need 2 teams and should prepare accordingly I think.

All this said, Tyranitar will still do a lot of damage from Bite, but you will probably need to swap out since most people probably won't be able to do it with 6 Tyranitars in a normal sized raid group...