I can see you

The easiest place to do this at is the Faith Camii landmark. There are plenty of guards at the landmark and at the Janissaries camp next to it. All you need to do is make at least 5 guards chase you, use a smoke bomb, turn on your Eagle Vision, and kill the guards with your Hidden Blades. If you're planning to do this with the "Lightning Strikes" achievement/trophy, you should try to get maybe 8 guards to chase you so you can easily get double assassinations with your Hidden Blades.