Max it out to 39 or 40. I maxed mine out. It's 86iv but only 14 attack. It's still mewtwo 300 base attack. I would trade mine for yours. Psychic is also good. It just depends what u want to use it for. U going to solo alakazam and gengar with it? Machamp? Gyms? Generalist dps for raids? Don't know how long the wait it's going to be til u can get another chance at one. I got mine in Sept but never got another pass. In my area they don't give a pass to people who already have one and recently they been giving it to people level 30 and below here. I'm not lucky enough to have more than one so I use ctm like nothing going from shadow ball to psychic back to shadow ball. Just be happy u got one. Many are not as fortunate as us and have one. Also not everyone has 82iv+ mewtwo.
Mewtwo Speculation
I now have a 15 Attack Mewtwo but it's 84% and will require a large amount of CTM for Shadow Ball.
Should I power it up, or count on lots of Mewtwos?
How long do you think they'll be around? And how generous might future EX passes be?
I have lots of Rare Candies but I also have lots of other legendaries that are looking greedily at them.

Answers
Pyschic is good for 2 man victreebel and machamps. I change it when I'm going to 2 man Vic and also fit a machamp raid in also. Shadow ball is okay against machamp but not as good. As long it has confusion. Psychic or shadow ball is good against gengar. Shadow is better for sure tho. Against alakazam pc shadow ball is best.
Sometimes I just use FB Gengars 2100 to 2150 for gym stuff; It'll autoselect if Blissey has Hyperbeam. However not that much to optimize moves for right now, 100 other ways to beat Machamp than Psychic or Shadowball Mewtwo. I guess Alakazam Raid boss but I don't see many of them and/or I ignore them
Shadow ball definitely outclasses it in neutral and when Pokemon is se against psychic and ghost. I thought so too but if psychic is se and shadow isn't psychic does just a little more damage. Like against victreebel those extra seconds help even tho it's not by much. But I think shadow ball has way way way more uses. Psychic is just for niche uses.
You do not have any better candidates for your rare candies and your stardust at the moment.
You will only get a better candidate if you catch a better Mewtwo. However, it is unlikely to happen soon, you need to get another ex-invite, catch the Mewtwo which is more likely to be worst than your current one.
And even when you get a better Mewtwo, this 84% one will still be your second best candidate for powering up. And you will still need to power it up. So why not powering it up now?
Yup max it out. U got 15 attack. U don't have to worry about breakpoints. Very low chance u might get another with 15 attack with higher iv. Maybe better iv and cp but not likely to roll 15 attack higher iv. Have fun with mewtwo :) pretty much output a ton of damage against every raid boss out there. Don't stop at 30. U need to max it out for the extra defense and stamina to survive longer. Plus u will have 315 attack. After u max it out u can also show it off to everyone at raids and here too ;)
Attack 15 is the only truly important stat for your Mewtwo. Defense and Stamina are very important, true, but dodging makes these stats less important. And since you can't put a Mewtwo on a gym as a defender...
Power it up. Power it up as far as you can - even if you don't CTM it right away, that Focus Blast can be deadly in a Tyranitar or Snorlax raid as a middle attacker.
And the chances of getting another EX pass are slim unless you're doing 3+ raids a day and not hitting the same gyms. The groups I raid with regularly hit between 20-30 different gyms and none of us yet have gotten an EX Pass; there has only been 1 EX raid anywhere near our area, and even though we've all done raids on that gym in the past we didn't get an EX Raid pass to the EX Raid. So don't count on getting another Mewtwo. Ever.
(if you do, congrats. Just don't count on it.)