How to replace frustration
Like I said, the quick and easy way to replace Frustration with one of this character's normal charge attacks allows you to cleanse them. For a small cost of Stardust and Candy, you'll take this shadow character and restore it again. Better in fact, as they will be purified, which will replace Frustration, increase their IVs overall, and allow them to be fueled for less stardust and candy in the future. But while they're better than a normal version of this Pokémon, they're not much better than their Shadow form.
Pokémon GO Shadow Pokémon have a significant attack bonus, which makes even the lowest Shadow IV version better than a purified or normal IV version. So you want to use Shadow Mewtwo or anything else that is powerful enough in these battles, but you need to replace Frustration with something that's actually useful. To do this, you have two options. The first is not really to replace frustration, but simply to make it irrevocable. By adding a second charged attack to any Shadow Pokémon you wish to use, you will unlock a normal attack and then be able to TM as you wish. It can be expensive, especially in Candy, so use it wisely.
Then you can make any changes you want to this second attack. Your best bet is an Elite ChargedTM, which can replace any frustration-free attack with one of that character's previously available (and legal) attacks. This usually costs a lot of money, but is also given as a reward for the Battle League Season Leaderboards or other events. If you have one, be sure to use it wisely to get yourself a Psystrike Mewtwo, Meteor Mash Metagross, or another equally valuable Pokémon.
Your only other option is to be patient and wait. During certain events in Pokémon GO, you will be able to use normal TM and Elite Charged to replace Frustration on your Shadow Pokémon. These events are rare and become more and more over time, so don't expect them to happen every month or two. The previous ones were hugely popular, with die-hard gamers shedding the frustration of all of their powerful Shadow Pokémon, but it led to some overkill options that Niantic is holding back on.
Here's how to replace Frustration in Pokémon GO, and a workaround if you can't wait to get it right.
