This can be done half-way through the Mini Golf Course on Day 2. It's easiest with 2 controllers.
Right after you learn the "High-Five to Heal" ability with
, you'll see an armored boar on a track just south of you. Damage in this game is shown by red/white icons on the bottom of the screen. Damage doesn't "count down", but builds up over time. Using 2 controllers to control both characters, stand in the way of this boar long enough to have both characters with 9 damage circles (you can get one point of damage on each character at the same time). After they have 9 points, move back up a little bit, then press
with both controllers to heal and unlock the achievement.