This technique guide you if you are fluent in the Rank 2 skill "Counter" and have enough Synchronization bars; one and a half rows of bars will do. When in battle, equip your hidden blade. With this equipped, all of your counters will score an instant kill. You can even instantly kill tougher ranked guards (for example, the Teutonic guards in Acre when hunting Sebrand). Also, most of the time using the hidden blade in battle will scare the last one or two (even three if you are lucky) guards, causing them to run away. However, because you do not have a weapon that is sufficient enough to block a full sized enemy blade, all incoming attacks will hurt you.
While in battle, if you see a guard you are fighting physically emote in any way (for example, taunting you or cowering in fear), press Low Profile Assassinate while facing them to score an instant kill without having to counter.
The best weapon for more than four guards or thugs is the Short Blade.