You're Invited!
What: Grunt Birthday Party - Official Xbox Live Co-Op Night
When: August 20th @ 8:00 PM Eastern
What to Bring: Your BR55 Battle Rifle
By far one of the most fun and icebreaking features of the Halo 3 co-op campaign is the Grunt Birthday Party skull, an unlockable "cheat" that produces a cloud of confetti and a soundbite of children yelling "yaaaay!" when a headshot is made on a Grunt. Not only is the effect disarming and hilarious, it's a great incentive to pop headshots on enemies that are normally consider easy fodder and mowed through at the expense of your meager ammo supply.
Grunt Birthday Party is one of Halo 3's Silver Skulls, and can be found on the level Crow's Nest. Part way through the level, you have to drop through a hole in the floor, where you receive one of those annoying Cortana messages that overlay the screen and buzz the crap out of your controller. When it's finished, you can see that you're in a service pipe of some kind that ends in a shaft that you will follow down to a checkpoint. Before you do that, however, get as close to the edge as you can without falling and carefully drop down, pushing yourself against the side of the shaft. If you do this right, you will end up just under the area you were just in, with the skull dead ahead. Grab it and hit that checkpoint, then save your game (and quit if you'd like).
If the above does not work for you, then grab a grav lift item at some point and use it to launch yourself back up from the bottom of the shaft.
Voila! A mere fifteen minutes, and you know have the most popular Halo 3 skull, according to the unofficial wiki. On top of the hilarity that this skull provides, popping Grunt heads actually render an explosive force that can damage nearby enemies. Now those Brutes that are surrounded by a squad of Grunts has that much more to be worried about.