I did this in Acre, rich district, but this method can really be used anywhere, except maybe the Kingdom or Masyef. I combines Topodude's method with guerrilla warfare. Basically I hopped around the roof tops assassinating one guard at a time on the roofs and "sniping" with throwing knives, occasionally dropping down to resupply. I did get into a few bigger fights, but using the dodge, block, and counter attack moves I won all of them.
Just try to take 1 guard at a time and then GTFO of the area that you killed him in, spending as little time on the ground as you can.