Now I need help

If anyone out there has beaten the ODST Firefight acheivement for beating a firefight mission on hard for over 4 sets, and if you did it on Lost Platoon, I need some tips to do it. My friends and I tried it about 5 times and we got to the 4th set, 3rd round, and 3 enemies left against 1 guy (who was an idiot, no offense). WE LOST!!!!!! Well anyway, if you have any tips about how to beat it on any map (preferably on Lost Platoon) post a comment on this post and I will look at it.