The 'Close Call' medal is obtained when a player with no shields and little health kills an enemy player, then manages to survive long enough to recharge their shields. This can easily be done on one of the bigger maps, such as Landfall. Use the large, open space on the exterior of the map to your advantage. With a Battle Rifle or DMR, you can engage long-ranged firefights where the enemy has a chance to strip your shields away. If you manage to kill them while your shields are low, run for cover and wait until your shields recharge. Repeat this method to obtain your second 'Close Call' medal. They do not need to be obtained in the same life and will most likely be obtained through natural game play.
Boosting Method: Since these medals are difficult to obtain "purposely" , here's an alternate solution. Have your boosting partner shoot you in the leg four times with a DMR to break your shields. Then, allow them to shoot you twice more in the leg to lower your health. Now quickly double melee strike them in order to kill them before your shields recharge. You will obtain the 'Close Call' medal. Repeat this method once more in the same match to earn your achievement.