To make this achievement a little less boring, combine it with 'Eagle's Challenge'. Attack a set of guards, killing all but one, and then hold RT to block him as you back your way through the city. He'll follow you, just swinging away. You move pretty slow, but eventually you will come across another set of guards who will set upon you. Kill all but one, and repeat. It took me about 7 minutes to kill 25 guards, unlocking 'Eagle's Challenge', and then you only have to sit and block for 2-3 minutes to get 'Eagle's Flight'.
And of course you can use counters or combos during this process to start knocking out those achievements as well.