After you thwart Wong's evil plans in Dragoon Castle, Will finishes his airplane and you fly to Mosque, then go to Beruga's Lab.
Go east to the next area. From there, follow the path until it splits, then go west. Check on the thing in this area to turn the lights on in the lab. Go east and then south and follow the path. There will be some pipes that you can climb under. You will crawl north to another path. Go northwest from there and there is a gap that you can crawl under. It's hard to see, but it's there. After crawling through that space, you will find a chest that contains SoulArmr.
Crawl back through the path and then go east. There are some Sentry robots around this area. Continue following the path. At the end, you will find another switch.
Go back the way you came until you return to the previous area, where you first were when you entered the lab. Go south. You will enter the elevator. Pull a switch (one of the orange ones) to make it go downward. Then go through the door at the end.
Follow the path. You will reach a conveyor belt. Walk across it and go south. Continue following the path until you reach an area with tanks of green liquid. Go west and then crawl into the hole in the north wall, just to the left of the last green vat.
After you crawl north through the long passageway, you will encounter some Dog Soldiers. Go east in this room and then go south. You will find a machine. You have to jump to get into the machine. It will teleport you. Follow the path from there. Ignore the next set of teleporters and just go north to the next area.
There is a switch here, so pull it. Then use the east exit. Push the barrier so you can go past, then go east until you can crawl through the crawlspace in the south wall. From there, follow the path until you get back to the elevator. Flip the switch on the right to go farther down.
Go north and you will encounter some Cubes. They can absorb you, so be sure to attack from a distance. Follow the path until you reach some conveyor belts. Jump on and go to the right, then jump into the first teleporter that you see. You will end up in another area. Go south and east from there, and go south into the room, where there are more cubes bouncing around. Go to the southwest corner of this room to get a DEF potion.
Go back to the teleporter that you came out of, and get in. Then go east to reach another teleporter. Jump in.
Go southeast to reach another teleporter. Jump in. The path to the next teleporter is straightforward. From there, go north and check on the machine. After Yomi talks to you, check on the machine again. It has some information that you can read if you're interested.
Now go back the way you came until you reach the elevator. Get in and flip the switch on the right to go farther down.
It's time to fight the Security Robot. Attack its feet. Avoid being below its left hand. If it opens its right hand, it will shoot bullets at the ground. You can easily avoid these with a dash attack. If the robot says "BURNER IGNITION", it is going to jump, so stay near the lower or middle part of the area to avoid letting it land on you. If one of the robot's feet is exploding, you don't need to attack it anymore, so focus on the others. After the feet are destroyed, the robot says "NOT FINISHED YET. ONE LAST TRICK" and sends some tiny robots to attack you. Kill these, and you're done.
After destroying the security robot, go through the north exit, then continue going north until there is a cut scene. Follow Beruga. Then talk to the zombie that Beruga is looking at. Then try to approach Beruga.
After a cut scene, you should talk to the people in the area, who will tell you about the starstones.