In Memory Block 1, after Al Mualim sends you to defend the fortress against the Templar attack, wait at the intersection/fork that is about 135 feet from the destination, just after the hill/path that leads to the Creed building. Guards will continuously respawn down the right fork. You can simply stand still and kill 25 guards, as your status will never leave "exposed", until you go to the destination.

Go to the entrance of Acre, but go through Kingdom. There is a base of soldiers. Start killing them but do not finish all of them. Start running to gather more. Kill a few and run again. Do this until you get the achievement. If you take long you may also unlock the "Eagle's Flight" achievement.