Well, at the beginning of a match it'd be better if you started out charging a Shadow Ball. Your opponent will have two options: let you charge it and have a very powerful move in your pocket so they can wait for you to approach, or run up to you in hopes of disrupting your charge session.
If they do let you charge it, you could shoot it immediately and run after it. This will make your opponent do a few things:
Shield
Jump
Roll
You can obviously read all of these and react appropriately. For instance, if they jump over the Shadow Ball, you can use any aerial or go for an empty shorthop and then fsmash, dsmash, or usmash. If they decide to shield it, you could go for a grab and decide what you want to do from there.
Really, I feel Mewtwo doesn't need to approach because of this. I know this is a long read, but I hope this helped! Correct me if you see anything I might have gotten incorrectly.