For your DACUS, you need to perform a dash, dash attack, then an Usmash all in the span of about 1/12 of a second. There are a variety of methods for doing it and even more controller configurations you could bring in, but the staple explanation is:
dash forwards with the control stick -> down on the c-stick (when set to smash) while rotating the control stick upwards -> press Z (when set to grab or attack).
Again, this all needs to be done in a very, very, very short amount of time. If you are dashing into an usmash, then you are not getting your dash attack out. If you dash and only the dash attack comes out, then you did not get the Usmash input made quickly enough.
I have nothing to say regarding Bdacus, I still have never bothered to learn it.