Kill three guards simultaneously with only throwing knives.
There are many places where there are multiple guards to kill. You want to find guards that don't have any armor on or very little (that way they die in one hit). All 3 guards have to die in one hit. Go to an area with at least 3 of the above guards and make sure you have knives equipped. Hold to select them on the right stick's wheel. Hold down when you have them equipped and you should see the guards light up. Now let go to kill them. If you have a bad position, you may press to cancel the throw and try a different angle.
If you still have not obtained the achievement by Sequence 4 Memory 1, then you can do it within the memory. Get to where you have 6/9 people knocked out and you'll come into an area with a ton of enemies. There are a good amount of unarmored ones.
if anyone struggles with this one: if you collect enough books to get the third masyaf key, you can access a mission as altair where you have unlimited throwing knives and dozens of spawning enemies... should be fairly easy to get it there
Like # 8 said, or if you don't wanna wait until you get to that point, just wreak some havoc get your templar awareness up, find an official to kill, but sneak up on him via the roof tops. They always have two guards with them. Hold down Y and when they're all lit up let em rip. Bleep blop.
This is for if the guards are not going down by one knife hit. Target one of them in the group and throw a thunder bomb at him (lethal pouch with impact shell, Arabic powder,& coal dust). Then hit 'em with the triple knives.
I just went back into the very starting mission where Ezio falls from the Masyaf castle, then he has to climb to get his stuff back, following the ghost of Altair, then once there u get ur throwing knives as well as your sword and blade, then a group of about 5 enemies comes at u after u run up, hold Y for a few seconds and let go, BAM 20 points for very little work lol.
Personally, I replayed DNA Sequence 1, Memory 2. Just after you've received your Hidden Blade again, you encounter 4 guards. Have you Throwing Knives set and hold Y to throw 3 knives. Hopefully, you will hit 3 of these 4 guards. :)