This requires understanding of different interactions and what Mewtwo actually wants to do at any given time.
An approach is something you do with the mindset of "I want to place this hitbox in such a way as to hit my opponent". It's raw, aggressive, and committed. Pressure is something you do with the intent of placing a hitbox where it (safely) interacts with your opponent's space, but actually
hitting them is not required and is potentially a product of your opponent responding poorly. Punishment is reactionary and increments your advantage in a meaningful way, though this does not always convert to damage. Gaining stage control is a form of punishment that can theoretically lead to bigger things through further interactions.
All three require you to know when to back off. Overextending potentially leaves you unable to make the most of your advantage or worse, allows the opponent to reverse the situation.
So here's the thing: Mewtwo doesn't really
approach. He's not built for that. Smash as a whole doesn't really allow you to just
go in and actually get something worthwhile for your trouble, though some characters can get away with it to an extent via strong movement/frame data/followups. So we focus on the latter two options: pressure and punishment.
There are two key elements of Mewtwo's kit to consider here:
-Hitboxes with moderate amounts of speed and disjoint, generally combined with above-average reach
-Flexible movement tools complimented by strong midrange burst mobility (teleport, wavedash)
Pressure uses movement to augment the use of his hitboxes and make them safer via things like hover cancels or fading back with hover, wavedash/land, or dash away. His strong wavedash in particular gives him the ability to move in and out of this space relatively quickly, akin to Luigi. While he lacks strong extended blockstrings, he can still exert limited shield pressure via potent mixups a la Peach as well.
Punishment involves baiting opponents with ambiguous movements and/or capitalizing on unsafe actions by your opponent, including at midrange via the aforementioned burst movement tools. You can then follow up in a flexible manner with Mewtwo's combo-friendly moveset.
Quoting myself from the Mewtwo skype chat:
movement is your #1 way of passively interacting with your opponent so that you can get a chance to actively interact with them (hit them). calling it baiting can be sort of misleading if you don't fully understand it though, because you're not so much going out there and saying "i'm going to make them try to hit me and miss" as you are observing patterns in their behavior, reacting to them, and taking advantage of them. mewtwo is very heavy on the mental game with the way he can play hot/cold with pressure vs punishment, so it's good to understand how you can throw them out of their comfort zone then create situations where they'll potentially make punishable errors
Make no mistake, this does not come without effort. Current Mewtwo play overemphasizes the (also very unoptimized) punish game in part because fluently integrating his pressure demands understanding, nuance, and proficiency in both the mental and technical aspects of the game.
tl;dr Don't look for one-size-fits-all options with which you can force things. Play to the situation. Use your tools to observe your opponent's behavior and respond accordingly.