im afraid im gunna agree. With enough experience, you will be able to figure out the strength and weaknesses of your opponents and henceforth come out with your own solution. Since mewtwo is almost always at a disadvantage in his matchups, every match needs to be played strategicly and differently. While in cases of falco and sheik, what they usually do, works on everyone.
The strategies could go very deep for each character matchup. But on the other hand, each character could have different playing styles even at a competitive level, and so an analysis for each character matchup might not apply in all cases.
In general... "Play More" is practically the answer to anything.
Once you are experience enough with your character and able to analyse a battle by yourself, you will find that you have become capable of playing lots of other characters, simply because "mindgames" and alot of other "advance tecniques" are all transferable skills. Being able to play a wide range of characters is usually an indication of how good a player is.
with that aside
Heres my analysis against marths.
You must stick to the ground. All aerial approaches will fail because mewtwo's aerial attacks have significantly less range than marth's aerials. So while staying on the ground, you are left with wavedashes and dashdances.
Spacing is key in almost all matchups, especially this one. Your goal however is not to make him miss a move. But to make him hit your shield with bad spacing. When opportunity comes, throws or dtilt to multiple fairs are your best choices for racking up damage. With that in mind, simple strategies like running up to him and suddenly holding up your shield works well.
When you need to edgeguard. Bairs work very well when hes in a distance. When hes aiming for sweetspot, dont try to dsmash because his UpB will hit you. Instead you could use the "roll towards ledge with light sheild aiming outwards" edgeguard. (you can ask me more about this). SUNC is also hard to do because you put yourself in danger of stage spiking.
when recovering from the ledge, watch his spacing carefully to figure out your best options in evading his fsmash. Sometimes it may mean that you should teleport horizontally directly onto the stage. Sometimes it means that you should sweetspot first then act accordingly. From grabing the ledge, dont do rising ledgehop aerials because you will always get fsmashed.
Upthrow is usually a good kill at around 120% on FD. If you are playing on a stage with platforms, try and stick to the ground.
And again, marth IS top tier, so if you loose, dont go too hard on yourself. Meanwhile, what is stated above probably are your best options.
Await taj for a more experienced answer.