After you get through the sewer and reach the Resistance Headquarters, go to the Empire Ruins.
Explore the Ruins
To reach the Empire Ruins, use the northeast exit of Northtown.
In the first area, go north. Phanna is at the steps and says some unkind words to Primm. Krissie comes and takes Phanna to the Resistance Headquarters where she'll get checked out by their doctor. Go north into the ruins.
Optional Orbs
You can get a Spear's Orb and a Bow's Orb if you go through the two doors on the upper level in this first big room. However, if you wait until you have defeated the boss here, you can go get those orbs without having to deal with any enemies.
Main Path
To continue on toward the ruins boss, go through the door on the first floor of the first room of the ruins.
Go north through this room that contains Ghouls and a Specter, and go through the north door.
In the next room, get past the Weepy Eyes to go west and down the stairs. There are three Grave Bats in the next room, then when you go south there is a Blue Drop, a Red Drop, and a Green Drop. Exit to the south.
There are two Ghouls just outside the door. Go all the way to the left, ignoring the door on the way that has a Robin Foot in front of it. The correct door is on the far left and has two Ghouls next to it.
In the next room, there is a Blue Drop. Go north through the door. There are Ghouls and an Imp in here. Go west and down the stairs.
Go along the right-hand wall. You reach a Wizard Eye near a door in the south wall. Go through that door.
The path through the next several rooms is straightforward. Eventually you reach a room with Blue Drops and a LA Funk. Step on the switch in here, then go back the way you came. In the room with the Wizard Eye, you can optionally go into the northwest room to get a Sword's Orb, or you can just come back later when the monsters are all gone. Otherwise, go to the northeast corner of the room, then go down the stairs to the left. Go south and step on the switch that was blocked by spikes before you stepped on the previous switch. A door will open behind you. Go through it.
The path through the next several rooms is straightforward. Eventually you reach a large room full of spikes. Follow the path to the end, where there is a switch. Step on it to lower all of the spikes. Go north and through the door in the north wall.
Follow the path through the next couple of rooms. Eventually you reach a Wizard Eye in front of double doors. Prepare for a boss fight (you might want to cast a Saber spell on all of your weapons) and go through the door.
Doom's Wall
You don't have the element that Doom's Wall is weak to (Lumina) but any of your highest-level spells will work, such as Air Blast. Target the central eye. Doom's Wall will start using Cave-In if the other eyes die or if it runs out of MP.
After the battle, you get a Whip's Orb. Go through the door in the north wall.
Go north onto the altar in the next room to find Dyluck. He tricks Primm into thinking that he's back to normal, then drains all of her energy. Randi hits him, which knocks some sense into him. He's immobilized but he says to go through the door behind the altar, so prepare for another boss battle, then go through the door behind the altar.
You'll find Thanatos standing over Primm's body. Dyluck will suddenly send his energy to Primm, reviving her. Then Thanatos will summon Vampire to attack you.
Vampire
You once again don't have the elemental that Vampire is weak to, so try using Air Blast. You can't harm Vampire with physical attacks if its wings are closed. Vampire can put your allies to sleep, so be prepared to heal or switch to a different lead character so you can keep fighting. Vampire has 2550 HP, so it can take a while to defeat.
After you defeat Vampire, you get a Boomerang's Orb.
Dyluck isn't there after you defeat Dyluck. Primm is distraught, but is confident that she'll find him.
Feel free to go through the ruins now and get the weapon orbs that you might have skipped before. You can reach one by going through the northwest door in the room where a switch was blocked by spikes. The other two can be reached from the first room of the ruins by going through the northwest and the northeast doors.
After defeating Vampire and getting the orbs from the Empire Ruins, you should have four glove's orbs, five bow's orbs, five boomerang's orbs, five sword's orbs, four javelin's orbs, five whip's orbs, four axe's orbs, and five spear's orbs.
Go to the Inn and go downstairs to get your weapons forged. Then restock your healing items.
When you're ready, go into the "coffee shop" (a.k.a. the Resistance Headquarters). Krissie will be there and you'll tell her about what happened. She'll say that the Emperor wants a truce and wants to have peace. Sounds great! And by "great" I mean "totally suspicious"! When you're ready, use the northwest exit of Northtown to go to Northtown Castle.