Grunts versus more Flood
When the level starts, kill the Flood chasing the Grunt, then run straight past the four Grunts and wipe out the Flood at the far end (two combat forms and a carrier). Approach the door to trigger Cortana's speech. Shortly afterwards, you'll have a few moments of amusement as you watch the grunty fools get the shock of their lives when the first wave of Flood appears, some of them dropping in from a hole in the ceiling. Take the opportunity to do your best Grunt impression and shout "Right behind you!". But don't laugh too long or you'll end up as Flood fodder too. You can go up close to watch, as the Flood tend to be completely distracted with the Grunts - who'll probably be running around screaming.