First and foremost: This does not have to be done in one round. It's your cumulative total over all your games.
This achievement is easily earned by playing the Anti-Terror gamemode.
AI Terrorirsts especially on the lower difficulties rarely finish of their kill and leave their enemies bleeding rather than dead, allowing you to heal your buddies back to life. Actual human enemies in the PvP gamemodes are rarely that forgiving..
The best gamemodes are the defensive ones because they take the longest, are relatively tough which increases the chances of your teammates dying (compared to some of the offensive variants) and your injured buddies are very likely to be conveniently in the same room as you are.
Alternatively....if you REALLY find yourself struggling to get this ( which is VERY unlikely), you can simply start a terror hunt or multiplayer game with a friend of yours and gun him down. ( You can also just gun down & revive randoms but please don't be that guy..)