The achievement 'Mega Chain Chomp' requires you to get an alternating combo of twenty blobbidees in Arcade mode. This might seem a little impossible because of mono-colored bonus rounds that pop up, which will ruin your alternating streak. The trick to the achievement is actually very easy, just go stand in one of the four corners when you enter the bonus stage and do not chomp on any blobbidees at all. This way the blobbidees will not accidentally run into you either, protecting your streak. Because time will not tick down during bonus stages (they automatically end after a few seconds), you can safely do this without disrupting your progress. When you enter the next stage just continue your alternate chomping spree :).