The best place to attract guards is by the restricted building in south-east Venice. There are generally two roving patrols and around a dozen guards in stationary positions. Just deck one upside the head and do a loop around the building and you'll have more than enough guards.
Just be careful with the sweeper achievement. It seems to want you to sweep EXACTLY five guards, which can be a problem if you're attracted a horde. Try getting their numbers down to five and then herd them into a nice confined area.
As for killing ten guards, try using smoke bombs and then just shanking them all with the hidden blade.

