In order for this achievement to unlock you will need to kill 3 enemies while your ally is in the downed state before their either bleed out, self-revive, or you revive them. The achievement itself is not that difficult as there are many areas where multiple enemies are grouped together giving you a good shot at allowing your ally to go down and then come in to get the kills.

A good spot to "boost" this achievement would be in Mission 3 New Vulcan > Chapter 5 when you go outside to catch the Gorn Lieutenant with the core. The first sequence where the Gorn land there will be 4 enemies that can be dispatched until the sequence forces a cutscene and you must move on.

~Int