I have about half of them done on the PS3 version. What I do is play it on Hero and get a few kills in the first min then hold block and either read neogaf on my ipad or do anything else for the other 2mins.
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
On Hero the cpu may get 2 kills in the 3 mins so anything in the 5-6 kill range for you works. I started doing this about halfway through my Vita plat run. Once you get past the first few matches and get to the ones not timed your home free.
Speaking of Vita I somehow managed to get mine caught in the car door when I closed it. It was in my coat pocket. I was lucky as the right stick took all the force and now has a dent. The stick is fine after a quick test on Uncharted. If it was the screen I would of been in trouble.
Speaking of Uncharted the .99 cent treasure map DLC is awesome for anyone going for all the treasures/photos. A small map is in the pause screen and it shows the whole level and where everything is located! I was using a video walkthough but hated switching from my Vita to ipad or PC. Easier to play at work or in bed now too!