Mission: The Storm

Effect: "Pull pin. Count to three. Throw." -- Every NPC carries two grenades and throws them often.

You can get this by performing a grenade jump off a warthog, or grab a ghost and go back to the ramp from which you first came out, using the ghost to push a big square crate back over to the front of the silo pod that the skull is on top of, then use the box as a place to jump from.