Southeast Burial Mound note: Earth, Fire, Water, Air.
Northwest Burial Mound note: Water, Air, Fire, Earth
Those are the codes needed for the respective mounds. But the levers activate elemental discharges in the other mound. The complicating factor is that it is not the matching element. You need to figure out what lever triggers what discharge, and the only way to do that is to split your party in two.
SE Mound:
Earth -> Water produced in the NW mound
Fire -> Earth produced in the NW mound
Water -> Air produced in the NW mound
Air -> Fire produced in the NW mound
NW Mound:
Water -> Earth produced in the SE mound
Air -> Water produced in the SE mound
Fire -> Air produced in the SE mound
Earth -> Fire produced in the SE mound
So to trigger the SE mound which has the code EFWA, the person standing in the NW mound has to pull WEAF.
To trigger the NW mound which has the code WAFE, the person standing in the SE mound has to pull EWAF.
There are two different fights. Defeat the enemies that spawn and get the pendants. Go into the trap door. Place each pendant on the altar before the force field and they'll both vanish.
