After you have learned heal at level 5 and have teamed up with Erik, open up the quick menu with
and then hit
to use your heal. When doing this while your main hero and Erik are at full health you'll get a notice saying 'Nothing happens---everyone's as fit as a fiddle already!" and the achievement will pop!
*edit* According to Boost17 pressing the quick buttons doesn't always work, so you can also press
, scroll over to 'Misc." - then press 'Handy Heal All' and it should work then.