Knock the Prophet of Regret out of his throne
From Halopedia, the Halo wiki
The following steps are a guide to Blow the Prophet of Regret out of his throne.
Walkthrough
1. Get the Sputnik Skull.
2. Play through the level Regret on any difficulty.
3. Get any explosive weapons you can find. The Needler and Fuel Rod Cannon work best.
4. Blast the Prophet of Regret with the needler or fuel rod cannon until his gravity throne flips over.
5. The Prophet of Regret will shoot his Plasma Pistol at you. He will teleport after 10 seconds back to his gravity throne.
