This is a quick guide for the Hornet's Nest achievement, I did this on Mission 6, Return to the Tower.

First off, you want to reach the first top area with guards, put them to sleep, using a sleep dart or choking them then place them in a row. There's 4 guards right at this top area that can all be taken stealthily to make this less of a headache. Place the 4 guards in a line, activate your time bend, quickly switch to your crossbow and take them all out in a line. There's your achievement!