I found that the easiest way to get 25 or more guards in one area is the gates in front of Jerusalem. There are 8 guards at the Main Gate. Outside to the left there is a camp with about 10 guards. Also outside to the right is another 10 guards. Start a fight with any of the three groups. Kill a few but leave 3 or 4 as they would run away when their numbers dwindle. Once you got their attention simple run to the other two areas. This ensures you have more than enough to kill to get the achievement. If the battle ends simply reload the last save point and try again.