If you're confused by what this achievement's description means, look at your partner when he's defeated and many times he/she would have an A floating above. Hold down A button next to your downed partner and he/she would get up.
Don't be surprised if you don't see the A though.
Update: The revive is actually tied to a health pack that the player would have on hand each level. There's only one available unless you somehow get another along the way.
Just keep fighting through the campaign and be mindful of your partner and you should get this soon enough.