The Altus Tunnel is found in Altus Plateau. Can be found directly south from the Minor Erdtree in Altus Plateau. The Perfumer's Grotto can be seen from the Altus Tunnel; they look interconncted, but are not mutually accessible.

Take the lift down to the tunnel. There will be a Site of Grace. Leaving the Site of Grace, the first room on the right is a small cavern with 2 enemies within, 1 Sorcerer Miner and 1 Avionette Soldier. The room being small as it is a large AOE attack such as using Cannon of Haima can quickly clear the space safely. The treasure chest within will reward you with a Rune Arc. Moving past this room and following the main tunnel the next space will have a Sorcerer sat on the left side of the room by some sacks. Defeating these enemies will give you a chance to loot a Large Glintstone Scrap.  The room connected on the right has multiple Crystal Snail within as well as a body that will drop Crystal Dart with many Cracked Crystal stored further in the back.

After this first area, you will enter the next section of the tunnels.  You will walk into the main part of the tunnels where miners are working away at the crystals found in the walls. This room has multiple and different kinds of enemies so take the fight slowly to not get surrounded and swarmed. The Avionettes move fast to attack the enemies while the Sorcerers stand afar and on the wooden platforms above the main floor of the tunnel. This room has multiple Smithing Stones for the player to collect both on the ground floor and on the wooden platforms. Notice the Crystals embedded into the rocks as these are also lootable. Loot them to obtain Smithing Stone (5).

As you exit this area, you will enter an open space. Arteria Leaf can be found on a body at the end of the wooden platforms. From this room there is a small tunnel leading to a stone ledge with a connecting wooden balcony area where a crystal snail sits next to a body that can be looted for a Golden Rune (7). If you look up, there will be a Malformed Star that can hurl rocks in your direction. It hangs over giant logs with some lootable items on it. You can choose to clear this enemy with your ranged attacks, but this area is more easily accessed from Perfumer's Grotto.

The image above shows the view of the Altus Tunnel wooden platforms where you are standing from the Perfumer's Grotto. 

From this area, you can take the largest log up to the higher wooden platform to loot Arsenal Charm +1. Drop back down, move back into the tunnels and take through until you reach the large wooden doors.

The next room of the tunnel is the final room before the boss fight. Many dormant Crystal Snails can be found here before the two large wooden doors that lead to the final room of the tunnel.

BOSS BATTLE: CRYSTALIAN SPEAR  & CRYSTALIAN RINGBLADE

The two bosses are humanoid enemies made of crystal that are not much larger than the player, making the arena quite spacious. The fight can be made much more manageable by using Spirit Ashes to help draw the attention of the bosses as you work on dealing damage to the bosses. A good strategy is to single out and defeat them one at a time.

The Crystalians take reduced damage from all sources until their poise is broken, when they get up you will see that their bodies are visibly cracked and they start to take normal damage. If possible target the Ringblade boss first as they have a more difficult attack pattern to deal with. The Spear user repeatedly hops towards and stabs their spear at the direction of the player ending with a slower large hop giving players lots of time to kite them and an easily timed window to attack them. The Ringblade user is much more mobile and difficult to deal with, attacking from range as well as having a damaging and moving spinning attack. Defeating both bosses will reward the player with the Somberstone Miner's Bell Bearing (2) and 1800 Runes.