Repost from the old Xbox forum - Please help to move to the appropriate forum when they are being created.
Got this tips from another website and just tried it earlier myself - Works the first time!
- Must have acess to the Venice, San Marco District
- Must buy "Long Weapon Training" from the villa combat instructor (the guy by the ring) for 10k
- Take fast travel from any other area besides those in Venice to the San Marco District
- Look for the first guard patrol
- The patrol should have a soldier with a spear
- Assassinate all 4 in the group and grab the spear
- Move towards North around the area of the Campanile di San Marco and look towards the east you'll see 4 Brutes standing together guarding the gate.
- Engage the brutes ( 1 random soldier should attack with them making 5)
- Pull off the sweep by holding 'X' and release
- Achievement will unlocked once five guards fall down on the floor (not necessary at the same time)