This takes a while, but set difficulty on normal (or easy for you noobs), then complete the first set of skulls.

Once the bonus round starts grab the rocket launcher, sniper Rifle or Spartan laser.

When all the grunts jump from the ship, send a rocket to the center of all the grunts (mega points!).

You can also laser through the middle and get a whole bunch of kills that way or rapidly spine through them.

I got at least 100k points that way (on normal).