As you enter the area, a cutscene showing the depth of the Sanctum’s treachery will play. Weapons of war, Berserkers, litter this area. Berserkers possess tremendous power, especially when coupled with their Centaurion Blades. You should always remove the Blades the moment the Berserker uses Forge Blade or you will find yourself in a lot of trouble. Starting the battle with Haste if you have Sazh or a few buffs can help absorb some of the damage. The Berserker is also easily Staggered via Lightning attacks and can be Launched to keep it from forging its Blade.
Run forward and confront the first Berserkers then check the top right corner for a Treasure Sphere with a pair of Rigels (a weapon for Sazh which increases Magic but Sazh cannot Stagger enemies) inside. Pass through the door ahead then access the elevator down to the next area.