Take Robo to the future to complete the Origin of Machines sidequest.
Put Robo in the lead of your party, and equip everyone with Shadow-absorbing armor.
Fly to Geno Dome. It is an island near the Northern Ruins were in 600 and 1000.
Go inside and check on the screen to the left. Go through the door, and if you want to leave the dome after this, go back and the door will be open again.
The conveyor belt leads to 5 fights, and with the Shadow-absorbing armor equipped, you can just use physical attacks, healing the unprotected party member as needed.
When you are done, exit north. If you want to gain some levels, go all the way right and there's a sparkle that takes you to a Dust Chute, which brings you back to the beginning of Geno Dome, letting you do the conveyor belt over again, which yields a lot of experience. After you beat Geno Dome's boss, you won't be able to use the conveyor belt again.
For reference, the total EXP you gain from the conveyor belt is 8286, the total skill points are 144, and the total G are 10,200.
After the conveyor belt room, go left, then up the passage. Watch and follow the robot. When it makes the door open, hurry through before the robot goes in. Open the chests and go back out.
Go back to where you saw the robot charge itself, and fight the enemies and open the chest. You can check on the computer for some information. One of the things it will tell you is that you need to collect two Poyozo Dolls to get to the boss.
For the first doll, go through the door to the left, and up and right. Beat the Laser Guards and then go up to the switches in back. Hit the left and right switch, leaving the middle green.
This unlocks the energy-activated pod. Energy-activate it (the charging pod is to the left of the computer), then go through the door that opens and get the doll. Wait a moment and then check behind where it was for a Speed Tab.
Now go to the bottom left of this screen and fight the two Debuggests. From there, go down and all the way right, and through the door around where the Dust Chute sparkle is. Take the elevator up.
Save if you like, then go through the door above you. Before running up the hall, get the Power Tab at the bottom right. Then go up and fight. Go through the door and take the elevator down.
To your left is a secret passage. Inside it is hidden a Magic Tab. From the entrance of the passage, keep trying to go up and left. You will stop before you get through the passage (if not, you missed it and need to try again). Then get back into the main hidden passage and go left, then down to the switch in the wall. Hit it, then exit through the hidden passage.
Go down, fight, and hit the switch to get rid of the laser. Go all the way left, hit the switch to open the pod. Now you have to go all the way back to the energizing pod, and from there, run into the pod you just unlocked. This step can be tricky, so if you don't get it the first time, keep trying.
Once you unlock the door, go through, open the chests, then check on the robot to get it to move. When you leave this room, the robot will follow you. It's slow and can get stuck near walls, so go slow and be patient. Take it to the bottom left of the room, where the other robot is, and they'll face off. Open the chest, then get the doll.
Now take the elevator near the Dust Chute entrance. Save, fight the Laser Guards below, and open the chest. Then heal up Robo completely and have him equipped with the Red Mail. If you want things more interesting, put Lucca in your party as well. Then go left, and out comes a pink girl Robo.
You actually don't have to attack Atropos at all to win this fight. She will go through a specific set of moves: RocketPunch, Cure Beam, Laser Spin, Atropos Tackle, Cure Beam, Uzzi Punch, and then she will do Area Bomb six times. After the sixth time, she will say "I--" and will explode, damaging you a small amount. Then the battle will end.
The ribbon from Atropos is not an accessory, but rather a permanent addition that boosts Robo's stats. After Atropos's sad end, go up and walk towards that switch to have Robo jump up and open the way. Go into the room and get the items. (There's a Tab in the bottom right corner).
Exit this room, then go left and follow the path, going down the ladder first and getting the chest on the way. Then go up through the door, and watch a terrifying scene... the robots are killing people, assembly-line style. Let's put a stop to this.
Exit bottom right for a chest. Then go back to that ladder you came down, and go up it. From there go up the passage above you. Kill the Laser Guards, then go up and kill more.
Place the dolls on the green spots nearby, watch the gate open, then go back and use a Shelter and save since you're about to fight.
For this battle I used Robo, Ayla, and Frog. I used speed boosting accessories on two of them, the Amulet on one, the Vigil Hat on another, and the Sight Cap on the last one. I also had the Black Mail and Black Vest equipped.
You can Charm a Blue Mail from MotherBrain's head. The Displays heal MotherBrain frequently, BUT DON'T KILL ALL OF THEM, JUST KILL TWO. If you kill all of them, MotherBrain gets progressively stronger and harder to beat, and uses an increasingly painful attack every round. So, leave one of the Displays intact. If anyone gets Chaosed (which they shouldn't if you followed my advice above), use a Heal on that person immediately so he or she won't accidentally kill the last Display. With the party above, I just used Robo and Ayla's Spin Kick, and Frog's Leap Slash until MotherBrain died. You can also use Robo and Ayla's Boogie to hopefully cast Stop on the last remaining Display, preventing it from healing MotherBrain and making the battle not take forever.
Now you have Robo's Terra Arm and Crisis Arm, and the Dome is shut down and you can't get in anymore.
A note about the Crisis Arm: the damage that it does depends on Robo's current HP. Specifically, it depends on the ones digit of his current HP. The higher this number is, the better. If his HP ends in a 2, he will do roughly normal damage. If it ends in a 1, he will do less damage, and if it ends in a 0, he will miss. Anything higher than 2 will do higher than normal damage.