In Brotherhood the fight missions for Mercenaries have a very short time limit on them. What do you need to do in order to win the fight in less the 10 seconds? I been trying and trying and can't seem to be able to do any significant damage within 10 seconds.
How I beat this challenge: equip your throwing stars (or dagger), and hold down the attack button for a few seconds. Ezio will crouch. Release the attack button and Ezio will whirl 5 knives at enemies. If there are at least 5 enemies within reach, they should get hit and die and you'll get the completion message. Done within 1 second!
Honestly, this part is extremely frustrating as your best bet for beating them in the alloted time is hoping that the game decides you are going to do a quick finisher combo instead of whacking all the health off an enemy one chunk at a time. I found just mashing punch and countering only when necessary to work the best and was able to complete all the challenges pretty quickly.
Most of it is just about luck I found.
All guards are killed instantly with combo kills ("kill streaks") and counter kills.
To get this achievement, first find 5 enemies. With the exception of Papal Guards, you may have to lure a group together. You can initiate a fight by tackling someone.
After you have the guards, remember every kind of guard will be counter-killed with the Hidden Blade(s), including the Seekers. After counter-killing one, you can use the killstreak to instantly kill the next guard, and the guard after that, until you reach 5 enemies killed.
I tried equipping my throwing knifes & holding down the punch button (square 4 ps3) but all holding down punch did was throw dust in their face which makes them stop & hold their head for a sec but after a couple times i noticed if i hit 1 right after throwing dust in their face Ezio will do a 1 hit k/o do this 2 all of them & once u get to 4 try to take out the one on the far left first he's the one that'll dodge ur attack everytime. Hope this helps!