To get far in Firefight on Last Platoon, wait until a chopper appears.

Then hijack it and kill the driver.

Then go to where the Spartan laser is and grab it.

Then get back in the chopper and start to kill everyone.

When a wraith comes get out and use it on the wraith to kill it easily.

If you don't miss a shot of the Spartan laser you should have plenty of ammo left to kill all of the wraiths in that round.
When it says weapon drop, switch your Spartan laser with the new one and do every thing again, and again, and again.

Note: it might not work if you miss with the Spartan laser or if you pick a hard difficulty.