Vivi heals her RCV each turn. Laboon makes it so if you have over 50% of your total health, you will survive then ext attack if it would kill you. The teamwork means Laboon saves you, and Vivi heals you above that 50% mark each turn so you will always be saved.
So as long as neither unit is locked, as long as there is only one enemy on the field you are literally invincible. Mihawk's 25,000 damage attack cannot kill you because of Laboon, for instance (though this requires a Princess Vivi at least level 72 to heal enough to overcome Mihawk's damage each turn.
So the only trick is managing to kill stages with multiple enemies. People generally stock their teams with Vivi, Laboon, and crabs (decent damage, low health). You want to keep your total health down else the amount needed for laboon to save you will be too high for Vivi to heal.