This solution contains the two earliest opportunities to get the rope arrow kills needed towards this achievement during your single-player campaign.
As a precaution, once you come across these areas you should save again but on a different save slot. Press the Menu/Start button and go to "Change Save Slot." This way you can try again if you fail as the game autosaves often. If you do fail and it autosaves, you must choose the option "Exit to Main Menu" and press "Continue" on the other file, otherwise, it will overwrite the save file. It's a good idea to save on all three save slots from time to time.
This first area is the same area where you acquire the rope arrow. Which is right after you fail to rescue the first pilot from the down rescue plane. As soon as you get hold of this new weapon there will be 5 enemies right above you. The first two you take out won't count, but the next three will. Try to get all three. Each guy will probably take 2 or 3 rope arrows before they come down.
Kuddlesnot Youtube video "#6 - Tomb Raider 100%: Return to the Village" at 12:00 shows you the location.
This next area has 8 enemies that can be prompted to get near the edge of the cliff. It may be a bit difficult to get all 5 kills here, but it's definitely possible. The difficult part is getting them to follow you to the edge of the cliff and not getting killed in the process. This area is located soon after you receive a call from Sam in which the guards find out that Sam has taken their radio. You will ride down a zip line and crash down into the roof a wooden shelter. There will be 3 guards trying to fix a fuel leak. Run past them to the right and go straight to the edge of a cliff. Take cover behind the wooden barrier and get their attention. To make this a little easier, while there take some shots at them, but don't kill them. Eventually, you will get them to come close enough to the wooden barrier that they will follow you to the edge of the cliff. Once they do, run across the wooden beam to the other cliff where the ammo boxes are. Stand your ground and take out anyone who shows up with your rope arrows as quickly as possible. There will be 5 more enemies coming up to get you once you take these 3 out. You will have to get them to follow you to the edge as well otherwise, they will shoot you from below. Rinse and repeat. Kill a few straight away if needed to make things easier.
Kuddlesnot Youtube video "#8 - Tomb Raider 100%: A Shanty Town Adventure" at 1:50 shows you the location.
Overall Kuddlesnot Tomb Raider videos give great advice in dealing with all the achievements.