Hello!
Actions of this kind must be taken with a constructor; if it's the wrong wall in the wrong place, things can go badly.
And no, you shouldn't attempt to do it yourself; hire a concrete cutter who can make a nice opening after you've got it approved by the constructor. As you mentioned, you might need bracing.
Don't forget that changes to load-bearing structures require a building notification, precisely so you don't cut in the wrong place and have the house collapse on you.
Contact your local building office for more information.
Regards: Torpalainen