Grats on the pass! Let me pass you some of my experiences:
- I've done 3 in the past. For the most part we used big groups, and for one of them we split and did it with 8 people.
- Mewtwo has been soloed with as few as 3 people, and can be done with 5-6 level 30's, depending on the moveset. Confusion makes Mewtwo harder. Focus Blast makes Mewtwo much harder.
- Before beginning, consider going in with a Pokemon to see what its moves are. After all, it's not like it wastes anything to dip your foot in the water. Let you team know what the moveset is to further optimize your counters. Counters will vary greatly depending on what moveset it knows. For example, you shouldn't be using Gengars if it doesn't know Psycho Cut, and you probably shouldn't be using Lugia if it doesn't know Focus Blast.
- The gym was under attack for nearly two hours before the raid started each time. The area isn't usually super busy, and the gym has an Instinct stranglehold on it (and I'm Mystic), so I only usually would get 6-7 balls. The last raid I managed to split the group into a Mystic and Valor group and got something like 10.
- WARNING: Mewtwo has a good catch rate (i think it's 6% base and 30% with bonuses, not sure and not checking right now), but don't underestimate it. A lot of people around me caught it first ball. Most people caught it at some point. All of my Mewtwos, however, have either been last or second to last ball, and this is with great throwing. It all depends on luck. I failed to catch my girlfriend's Mewtwo and my friend's as well. Don't let that freak you out, since they only ever missed one and chances are you'll catch it, but every ball might count.
- Lastly, try to coordinate with your team. Show up early to prepare. Charge up and bring external batteries if you have them. Lastly, wait for 15-25 minutes before attempting the boss, as people are ALWAYS late to the Ex-Raid. If you're unsure of your group size, you might need every player you can get.
Let me know if you have any more questions, and good luck~