See Party Missions for general overview of party missions.
First, use the gate at D7 to warp down into the crater. Souls of Ancient Warrior (L130 Undead) roamed around the crater. Kill the four Cerberus. The Cerberus are not active at the same time, so you should not need to engage more than 1 or 2 at a time.
When all four are dead, Sekhmet will spawn and a zone notice will say: You will not be able to come out of the fire hole... Sekhmet has a large area damage skill called [Volcano], so use [Whole Cancellation] to prevent it from using skills. Use [Magic Barrier] to negate magic damage as well.
After 5-10 minutes or so, there may be a zone-wide notice, saying: Strengthened by Ancient Cure Stone, Sekhmet's recovery rate has been maximized. When this happens, 5 Cure Stones will appear at the edge of the crater, destroy them as soon as possible to stop Sekhmet's enhanced regeneration. Return to attack Sekhmet until it is dead.
See also シャクミス 党.