Although you can grind this out anywhere, this can easily be done in the amusement park level on Easy. Once you get to the boss (dwarf on stilts), let your character get almost knocked out (the boss does a ton of damage). When you are almost out of health, switch to your other character.
Now wait for you AI partner to get hit and downed. To save them, walk up to them and tap the
button. Do this 3 times.
Now, you will notice that the achievement does not unlock right away. You must continue the fight and beat the boss before the achievement will unlock.
(If you haven't figured it out, you can set the toy car on the miniature racetrack and fire it at the boss to knock him down so you can attack).