Have your party at least at level 50. You can defeat Zeromus as soon as you complete the Stilshrine of Miriam dungeon and get through the scenes at Mt. Bur-Omisace afterwards. After defeating the Judge, exit the map where you killed him and speak to the Acolyte somewhere on the map to receive a Stone Of Condemner. Stock up on healing items as you cannot use magic during the battle with Zeromus. At the southern end of the Ward of Measure, touch the Waystone. You will be given an option to use the Stone Of Condemner; use it. You will be transported to the Throne Of Veiled Gods. Before opening the door to Zeromus, place positive status effects on your characters and tweak your Gambits to use items instead of magic to heal. You may want to do this later in the game when you have more powerful equipment and are at a higher level.

Have your party at least at level 50. After defeating Judge Bergan, you will automatically be teleported directly outside the area. In front of you will be an Acolyte pacing back and forth. Talk to him and he will ask you to find a "greater power" in the Stilshrine of Miriam, and give you a Stone of the Condemner. Teleport to the Shrine and go inside. Go to the Way Stone at the south end of the room you are in. You may have to fight two undead foes at the Way Stone, but if you can defeat them you can defeat the Esper. If you use the Way Stone, it will stop and say that the stone is having a reaction. Use it to be transferred to the room before the Esper. All Magicks are disabled in this battle. Before the battle, change all healing Magicks to items because you cannot use Dispel on him; you cannot get rid of his status Help. The Boss is also not alone. He is joined by a large army of Dark Lords. If you have Holy Motes, use them. After defeating about thirty Dark Lords, they will stop appearing. When he is very low on heath (about 30%), perform a Quickening Chain. Keep everyone above 50% HP, just to be safe from from the Esper's Gravity Magick.