Location
If you check the map you'll see that there's a small cave right beside the bridge. Cross the chunks of bridge to the other side, then get down into the valley surrounding the nearby Minor Erdtree. Follow the bridge south, then check along the rocks and cliffs to find the entrance to Altus Tunnel.
Altus Tunnel
Before you reach the Site of Grace you may notice that there are ledges below the elevator at the entrance to Altus Tunnel. You can hop down them if you want, but there's nothing to find this time, so... meh. The summoning pool is just down the hall from the Site of Grace.
Across the hall from the summoning pool is a room with a visible miner in it. Upon entering, however, a Marionette Soldier will drop down beside the doorway and attack. Either draw the spellcasting miner out of the room or destroy the Marionette Soldier as quickly as you can before attacking the miner. There's a Rune Arc inside a chest near the miner, and there's a Smithing Stone [5] behind a heap of crates in the rear of the room.
Down the hallway is another spellcasting miner. Kill him quickly, then look to your right. Inside the next room are a number of snails - you can kill most of them without hassle - as well as eight Crystal Darts, sitting beside a giant root.
Continue down the main tunnel. You'll emerge into a purple-blue cavern filled with worming miners. There's a platform ahead and on your left, and two Avionettes are standing on it. These things are a massive pain if given a chance to wind themselves up, so try to snipe them from the entrance. This will also prevent you from immediately aggravating the Avionette on the platform just above the entrance, as well as the miner on a platform up and on your left. Once the first two Avionettes are gone, deal with these other two threats.
The room is now (mostly) cleared. To the right of the entrance you'll find a Smithing Stone [5], and there's another Smithing Stone [5] below the left platform. The miner to the right of the platform is working on a third Smithing Stone [5], though he'll attack you if you grab the stone. Backstab him. Go up the stairs and follow the wooden walkways to a cave with another magical miner in it. Kill him and grab the two Arteria Leaves he was guarding. An Avionette will try to swoop in and ambush you here, so be ready to roll away from its attack.
Return to the ground floor. If you take a right down the passage with the big root sticking out of the ground you'll come to a chasm with a series of walkways above you. In the middle of the chasm is a Malformed Star, a massive, insectile creature that will hurl space rocks at you every couple of seconds. Kill the snail on your right, grab the Golden Rune [7] beside it, then take cover and shoot the Malformed Star from a distance. Depending on your level this might take no time at all, or... you could be here for a while. You can use the root twisted around the wooden platforms to go up, which will get you an Arsenal Charm +1, and down, where you'll find six Glintstone Scraps. Watch out for snails on the lower level.
(Don't sweat it if you can't kill the Malformed Star from here. It has a lot of health. You can get the items without dying pretty easily, so long as you move fast.)
Carry on east through the tunnels. The next room has a bunch of snails in it, all tucked into their shells. Take them out, then check the walls for two Smithing Stones [5], one of which is behind a root jutting out of the ground. All that's left past them is an ominous wooden door, and the bosses beyond.
Before you reach the Site of Grace you may notice that there are ledges below the elevator at the entrance to Altus Tunnel. You can hop down them if you want, but there's nothing to find this time, so... meh. The summoning pool is just down the hall from the Site of Grace.
Across the hall from the summoning pool is a room with a visible miner in it. Upon entering, however, a Marionette Soldier will drop down beside the doorway and attack. Either draw the spellcasting miner out of the room or destroy the Marionette Soldier as quickly as you can before attacking the miner. There's a Rune Arc inside a chest near the miner, and there's a Smithing Stone [5] behind a heap of crates in the rear of the room.
Down the hallway is another spellcasting miner. Kill him quickly, then look to your right. Inside the next room are a number of snails - you can kill most of them without hassle - as well as eight Crystal Darts, sitting beside a giant root.
Continue down the main tunnel. You'll emerge into a purple-blue cavern filled with worming miners. There's a platform ahead and on your left, and two Avionettes are standing on it. These things are a massive pain if given a chance to wind themselves up, so try to snipe them from the entrance. This will also prevent you from immediately aggravating the Avionette on the platform just above the entrance, as well as the miner on a platform up and on your left. Once the first two Avionettes are gone, deal with these other two threats.
The room is now (mostly) cleared. To the right of the entrance you'll find a Smithing Stone [5], and there's another Smithing Stone [5] below the left platform. The miner to the right of the platform is working on a third Smithing Stone [5], though he'll attack you if you grab the stone. Backstab him. Go up the stairs and follow the wooden walkways to a cave with another magical miner in it. Kill him and grab the two Arteria Leaves he was guarding. An Avionette will try to swoop in and ambush you here, so be ready to roll away from its attack.
Return to the ground floor. If you take a right down the passage with the big root sticking out of the ground you'll come to a chasm with a series of walkways above you. In the middle of the chasm is a Malformed Star, a massive, insectile creature that will hurl space rocks at you every couple of seconds. Kill the snail on your right, grab the Golden Rune [7] beside it, then take cover and shoot the Malformed Star from a distance. Depending on your level this might take no time at all, or... you could be here for a while. You can use the root twisted around the wooden platforms to go up, which will get you an Arsenal Charm +1, and down, where you'll find six Glintstone Scraps. Watch out for snails on the lower level.
(Don't sweat it if you can't kill the Malformed Star from here. It has a lot of health. You can get the items without dying pretty easily, so long as you move fast.)
Carry on east through the tunnels. The next room has a bunch of snails in it, all tucked into their shells. Take them out, then check the walls for two Smithing Stones [5], one of which is behind a root jutting out of the ground. All that's left past them is an ominous wooden door, and the bosses beyond.
Ah, yes, these guys again. Crystalians are always a bit of a pain to fight, and this pair - one wielding a spear, the other a large ringblade - can be quite a nuisance if you aren't carrying around a striking weapon that bypasses their poise. Still, this fight isn't that bad so long as you find a way to split them up. Here are some tips for doing just that:
- Pick a Crystalian and stick with that Crystalian until it's dead. Both Crystalians will wander over to you, but the Spear Crystalian is much more aggressive, and will bounce towards you constantly. It's likely to be your initial target as a melee fighter, though you can go for either if you're battling at a range. Regardless of your choice, keep moving around the room while attacking so the second Crystalian can't get in the way.
- Keep attacking. Yes, it looks like you're doing virtually no damage, but you are - you just need to break the Crystalian's poise. Once you've done enough attacks you'll stagger your target, and every attack that comes after will inflict a normal amount of damage. You'll also put the Crystalian into a lesser staggered state, allowing you to effectively stun lock the boss and beat it to death without reprisal.
- When possible, use spells or Ashes of War that direct themselves at targets. That way you can focus less on aiming and more on fleeing the attacks of the Crystalians. Ash of War: Glintblade Phalanx is one such example: Summon swords, let them smash into a Crystalian, retreat, summon swords again, repeat. Your enemies will be staggered before you know it.
- Summon help. Spirit Ashes are great in this fight. The more the better, so long as they're decently upgraded and can survive a few attacks. Neither Crystalian is particularly good at dealing with mobs of foes, and they'll be staggered by your summons before you know it.