Are you looking for strategy or how to get the elevator to go from the Apex to the ground floor? After accepting the mission, look for a statue on the roof (marked by a dark red dot on the map). He will bring another elevator up, which you can ride to the seventh floor. Find the statue on floor 7 to make the central elevator go to the apex. Then ride from the apex to the ground floor to make Mithridates appear. He uses Lightning attacks and Status ailments, so be ready to defend against both. Dispelga is useful if your whole team gets hammered with statuses.