I finished the last Might challenge by setting my Vita on a table with the screen upside down to me, so that my bottom thumbs can press the block bumper and the top is firmly against something so it doesn't slide away. I placed my first two fingers of both hands on each attack button, alternating presses as fast as I could (i.e. don't mash all fingers together but alternate paired fingers like a drum roll). When the meter was high enough, I pressed the bumper (at the bottom of the upside Vita) with my thumbs (while still alternating my fingers on attack buttons).
Does that make sense? I can re-explain if needed.