One of the reasons the Smash Box is the best controller for Smash Bros. Ultimate is how much access you have to buttons. Since you can use all of your fingers, you have the ability to press a lot of buttons in a short amount of time. And one of the most obvious scenarios to take advantage of this access is when mashing buttons matter: breaking out of grabs, being buried, and being stunned.
Luckily, HiGPS has created a guide on the most optimal mashing method on Smash Box for Smash Ultimate. It requires having a specific SOCD Resolution to work, so if you wish to set up your Smash Box for this, follow the instructions in the video below or use our guide here: Changing SOCD Resolutions on Smash Box.
In short, once you have Second Input Priority set for both the X- and Y-axis of your Smash Box, simply hold Up + Right and tap Down and Left to get the best mashing.
And if you're looking for more info on Smash Box or SSBU, check out the blog tags below or use the "Index" to find the most pertinent info.