How can I get the 'Messer Sandman' and 'Sweeper' achievements in Assassin's Creed 2?

Messer Sandman
I did this in Venice ... it is easy to do in Piazza San Marco ... bother some guards and run around just to make them come at you. Once you get a group of 10 guards throw a bomb to them to keep them steady. Then throw the sand at them.

Sweeper
Again, this is easy to do this one in Venice, just like the other one ... the only thing to keep in mind is that you have to steal a long weapon from a guard... get a little bit far from enemies then charge! Other nice place to do this one is inside the city of Forly. Repeat the technique of busting an enemy and then run until 10 or more follow you.

Good Luck!