Unexpected Ex-Raid Passes for Low-Level Trainers. Do We Need to Prepare?

Honestly, unless you feel the need to power things up, you can probably just show up and go in with the Mons you currently have. Typically, EX raids will draw a decent sized group of people, so you should have more than enough to take Mewtwo down. Also, for the most part, people don't tend to exclude others at EX raids, so you shouldn't have any issues getting in on the action with the others who show up.

My best advice to you is this: show up early to the raid, and try to chat with others there (so they know the two of you are also there for the raid). Often times, people will start the raid as soon as it hatches unless they're waiting on others they know will be arriving late.

Good luck, and I hope both you and your kid catch Mewtwo! Be sure to take your time on the catch sequence and use the trick to freeze the circle in order to minimize the number of balls that get dodged or batted away.

Really the best option here is to either level the Dragonite up or try to complete a Tyranitar raid prior to your EX-Raid. Tyranitar is really something else, the best answer for Mewtwo and really good all-around. Even a level 20 Tyranitar straight from a raid outperforms most counters.

Outside of that, I would stick with the Dragonite, since they're pretty useful all around. The rest of those counters wouldn't survive long enough to do any real damage outside of Houndoom, and Houndoom is really only a decent counter for Mewtwo and nothing else. If you're going to use a Houndoom, you're better off evolving a weather-boosted, high level low IV one caught during Sunny or Foggy weather.

Show up 15-30min early. Chat with people. Join a larger group. Your Pokémon won’t put a dent on Mewtwo so others will have to carry you. Most importantly, go enjoy the game :)

On the other hand, Dragonite is the safest to power up. Will help in many things. Other Pokémon’s you power up will be waste of resources.

Level up you filthy casuals.

Seriously, just turn up and look for a group on their phones. Introduce yourself and you are good to go. If the group has less than 15 trainers, your pokemon may all die, do not leave the group. Press the green "rejoin battle" button and go back in with whatever auto-selection team you are given, to keep fighting.

When it comes to catching Mewtwo be sure to swipe to the right and use "Golden" raspberries for your best chance to catch the beastie.

With the new system of distributing EX-passes, probably a lot of casual players will get one for the first time.

One tip not previously mentioned here as far as I can see: Set up a second raid team in advance. It saves a few seconds on reloading, while still allowing you to bring in a team better than an autoselected one.

Powering all that up may get you 1 more ball at most.

Chatting with others, and splitting into optimal sized parties of the same team colour could get you up several more balls.