To get savior kills, you must kill any enemy that is engaging your teammate. You will have to kill the enemy before they take out your teammate. Doing this will grant you an extra 300xp. If you hold you will be able to follow one of your teammates around. Just cover them while they are getting shot at and you should get credit towards one savior kill. Do this 4 more times before the match is over and the achievement will be yours
Note: You will know you got the Savior kill when it pops up under your kill score on the top portion of your screen.
Note: This does NOT have to be done in one game.