How do I beat the "Kill 5 guards within 10 seconds" mercenary challenge?
Solution 1:
I've found that keep pressing your attack and kick buttons alternating them every so often allows you to do combo moves. I was able to do this for all five fights.
Solution 2:
Found a really great way: taunt them and then counter. Most of the time you will get a 1-hit-kill.
Solution 3:
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!
Solution 4:
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.