The spawn in Hong Kong is not infinite. You can get roughly 30 kills with the punk busters before they stop spawning. The check point is right at this spot and after you kill the rats just simply reload and do it again until the achievement pops. Also for the KNOCK 20 EVO'S OUT OF THE WORLD achievement you can utilize the same method as above. At the platforms in the beginning of pocket dimension you'll come to one that gives you a check point. Jump to the next platform and jump stomp the evo's off the side and then reload.
Last edited by spockroxfaces; 03-08-2012 at 05:32 AM.