As I hadn't completed the game and was still leisurely playing through, I was looking for a solution using only the first map. What I have found is a solution where you can easily get 5 kills in 2 minutes. What you require is Shadow Strike.
I have included a video which will allow you to see the path I have taken. Essentially, fast travel to the bottom left tower and turn around. Use
to aim at the first archer which is patrolling the tower in the fortress you can see from the tower. Shadow Strike (
) to him and immediately hold
. Sometimes you will pick up the Elf Shot and you have to hold
again. If not you will grab the Orc and can then dominate him with
. What option you choose then is not important.
Once he has been dealt with, run up the ropes to the pillar and use your bow to get the second Orc into your scope. Shadow Strike to him and repeat the process. Note that sometimes this Orc did not spawn for me. A bit further in the fortress, patrolling the right hand side of the fortress, is the third archer. You can easily spot him from the second archer and he will even, sometimes, be aware of you. But this isn't a problem as you can quickly take him out with the Shadow Strike. For the fourth and fifth archer you need to Shadow Strike across the gap between the two fortress parts. You might need to wait a bit for the Orc to get into position. If you executed the first three Shadow Strikes flawlessly then the Orc should already be in position.
Having a rune equipped which will replenish your Focus on a head explosion or other action you are doing (like a regular kill) will help you to keep using Shadow Strike. After you are done with the fifth archer, open your map and fast travel again. This will reset the archers. Using this solution you can easily get this achievement out of the way in less than half an hour even if you haven't advanced to the second map yet and have done most of the easy missions which let you pick off archers.
Edit 8 November: Relinked video to different channel.