So my first Mewtwo EX raid went pretty well, but not all experiences at this location were great.
Our group (Mystic) was pretty organized. One of our local L40 guys was our gathering spot. First 30+ minutes was us trying to tear the gym down before the hatch to get gym control. No such luck, but it was fun to try. More experienced players were squaring away the less experienced (I personally went through two other pokedex to select the best counters). One of us went in to "solo" the raid so that we could determine the moveset of the raid boss beforehand. I don't remember the quick move now, but the charge move was Shadow Ball, so I mobilized my Tyranitar army (5 L30+ B/C, with an 3400-ish Dragonite as my 6th).
We had 17 people and pretty easily convinced our group to split. The lower-levels and the people that needed to leave went first (about 8 total) and multiple (like 5) higher level players went in with them and got them to a point where they couldn't fail, after which the higher-level players dropped off and joined the second-wave.
In the second-wave, we mowed Mewtwo down easily with 9 people. Our core was like 4 of us at L39+ (I was in this core), while the rest were around the mid-30's. We could have dropped down a player or two and still made it no problem. I think I got +11 on premier balls because of not getting gym control and still only delivering +2 on damage due to how many players we had in our raid party.
Other groups weren't running so smoothly. My wife (Valor) told me that Valor wouldn't go in without EVERY SINGLE VALOR going at once. Their raid party was like 17 people total, which dropped their damage to +0 or +1. My wife ended up with +10 premier balls and caught Mewtwo on her very last ball.
Instinct also apparently had issues organizing and people didn't want to split even though they were apparently over 20. I don't know much about the specifics of what happened in their grouping though, but I do know that some people were getting heated.
Regarding the capture, Mewtwo was pretty easy. I had watched quite a bit of analysis on his attack moves and could threw greats on him pretty easily. I got him pretty early on too, 3rd ball, so I didn't even have to waste many of my precious golden berries.
Best part is in the picture below though!
Yup, got a hundo! I knew it was a 100 IV too, so it was a bit nerve-wracking, and I didn't tell anyone until I caught it. Already dumped a bunch of rare-candy in it and powered it up some (and will be doing more from this upcoming dust event).