The easiest place to get this achievement, I felt, was by loading the mission The Loyalists. If you want, you can improve your movement speed by buying the Agility II power, equipping your Fleet Fighter bone charm, and using Blink II, However, I was able to do this with weapons sheathed just by running, breaking line of sight and hiding, as follows:
Right in the beginning of the level, as soon as you leave the apartment, there will be three guards talking in front of the pub. Immediately run straight at them, getting their attention, and into the front door of the pub. Inside the front door to the pub will be two more guards. Those are the five you need to notice you, right there at the beginning of the level.
Turn right and quickly run up to the second floor of the pub. Run past Pendleton and Haverlock's office into the Barracks (the room with all the bunk beds) and then into the bathroom. Go stealth and close the door behind you. Then just wait. When they can't find you, the achievement will pop.
That's all there is to it! By the way, you can also use this strategy to get five guards to notice you for the Cleaner achievement.
The Cleaner achievement in Dishonored worth 12 pointsFight with 5 enemies at once, without any of them surviving If this solution helped you, please give it a thumbs up! If not, leave a comment and I will try to answer any questions you may have.