Pushing mongo means pushing with your front foot. If you are using regular stance this will be done with the X button. If you are using goofy stance this will be done with the A button.
You can hold the button for multiple pushes however the achievement won't register every single push if you do hold it. It will always register the push if you tap the button every time you want to do a push - when using the tapping method your front foot will sometimes glitch and not go back onto the board properly. You can foot brake using B to reset this.
This will also register if you're pushing switch stance mongo. I will do a video at some point and edit it in to this solution.