Army of Two: The 40th Day

you can also get to a part where there are no enemies and tell your partner to hold then just shoot him down on your own and then revive him

Revive a fallen ally 10 times. To do this when your ally has been downed, simply get over to them, press 'A' to grab them, drag them to safety and then hold 'A' to heal them. It is also useful to know that your downed ally can slowly move themselves in to better cover while downed. They can also use their pistol to shoot enemies. They have a limited amount of time in which to be revived...And finally...enemies do sometimes tend to home in on your downed allies to snap their necks, so do keep an eye on them!

A fairly easy way to get this achievement is the way i actually got it. I was playing solo and was having a difficult time getting heals, but what i discovered by accident is that if you shoot the russian in the head during the mission "Just blow the place to hell" and get to him before your partner you can heal him and get the achievement. is also good cause it only takes a single shot to down him and usually theres not much cross fire between you and enemies.