You can NIL on the top platform of battlefield by just jumping and double jumping at the right time. It's more convenient if you are near the middle of the stage. I like to do NIL up-tilt, grab, or up-b. Even dash attack is actually super good if they went to high to continue a combo and they try to drift beside the platform.
Second hit of f-smash is really strong. However, at 75%, I think fully charged d-smash will kill, anyway (probably depends on stage), and is easier to space. The f-smash might be a bit faster if you are tight for time, though. You can also just try a strong hit of dair.