You have to backtrack to the first save point near the exit on Tatroi’s side. Since you already explored the cave at this point, you just need to follow the directions below to reach the boss.
1. The door here is one way only so you’ll need to find your way around. Continue following the path until you reach a split.
2. Take the path to the left and follow the path you reach the four-way intersection.
3. Take the path south and exit to the right to reach the savepoint.
4. Prepare your party, save and head south for a boss battle.
There’s a healing statue near the save point. You can use this spot to familiarize with the enemies inside the cave and conveniently train as well, since there’s only three fighters in your party if you recruited Ashlay. Once you’re conveniently leveled up with all the best gears you can find at the moment, then you can comfortably explore the cave. Since you've reached Astral City after Ashlay joins, this will be the first time you're exploring this cave so follow the instructions below to get all the loot.
From the save point, follow the straightforward path until you reach a fork in the path. Take the southern path and continue following it to reach a dead end with three treasure chests. Loot the Amulet of Antivenin, Sacrificial Doll, ?Weapon from it. Backtrack to the fork and take the left path this time.
In the next fork, take the path to the right to find a chest containing a Necklace. Backtrack to the previous screen and head south. Continue past the bridge until you reach a four-way split. The path to the east leads to a dead end with a mineral ore and a chest on a ledge that’s out of your reach for now. The path to the NW leads to a chest containing a Gladius. (dagger-type weapon)
Take the path south then continue east to find a savepoint. After saving your game, don’t head south yet since it will trigger a boss battle. For now, head north to find a split on the path with an ore. Go west to reach the chest containing a ?Item.
Backtrack to the previous screen then head north. Cross the wooden bridge and follow the path to reach a room with three doors and a lever in the middle. Loot the Sinclair from the chest in the middle.
Pull the switch once to open the left door. Enter the door to find 1200 Fol and a mineral ore. Return to the switch room and pull the switch twice to open the door to the right and loot the Flare Bomb inside.
Pull the switch twice again to open the door to the north. There are two mineral ores here. The door to the north will loop you back to the savepoint and healing statue near the exit of the cave leading to Astral. Head there if you want to heal up or resupply; otherwise, backtrack to the previous save point and prepare for a boss battle.
After defeating the boss, you’ll be automatically back in town. Watch the following scenes. In the morning, you’ll obtain Cinderella Glass, Lunar Talisman and 200 skill points. Your next destination is Purgatorium. Save your game and head back to Tatroi taking the ship ride.