Are there any easy methods for unlocking the First Contact achievement?

All me and my friends had to do was hop in the Bulldog with the mounted gun and shoot down all the ships. None of them seem to try to escape, so just blast away.

There are 20 alien spacecraft in the final chase scene to take down. If you're struggling to do so, first try shooting in short bursts so that you don't have to waste time cooling down the turret. To see whether you're on track, by the time you enter the final tunnel, you will have taken down 14 full-size spacecraft. The final six ships are smaller than the others.

It's a good idea to reset and try from checkpoint if the achievement doesn't pop by the end of the tunnel sequence. If you finish the level, you'll have to play through the whole DLC to get another try.