Throughout Copperbell Mines, you will find Firesand either on the ground or dropped from certain enemies. Two of them are consumed at each Powder Chamber to enable the Blasting Device, which will blast a hole in the wall, allowing you to travel deeper into Copperbell. At the time of this writing, you need to get every single Firesand, as there are no extra.
In addition to the Firesand, the tank needs to be alert. At two different points in the dungeon, a Gigas will break out of a wall. It can wreck some havoc if the tank doesn't notice it quickly.
When entering The Screaming Dark, the location of the first boss, he is nowhere to be found. Instead, you will find a low HP Spriggan Sifter. Engaging the Spriggan will start the encounter, causing more Spriggan to began spawning and the door to lock.
In addition to the Spriggan, bombs called Flambeau will spawn occasionally. These should be your primary target, as Self-Destruct is capable of one-shotting any of the players.
Once everyone has fallen asleep from bordem, Kottos decides to make his move. He will appear on the platform above and jump down. However, even he doesn't use any abilities worth noting. Once he's been put down, grab the key and coffer and head through the Sealed Blasting Door.
This is quite an interesting boss. If it were tuned to be much more difficult, I would love this fight. However, as it stands, it's very easy.
To begin, the tank should grab Ichorous Ire and, if you would like, begin to kite it in a circle. Despite instict, you should not DPS the Ichorous Ire. It takes reduced damage. Instead, one of the other players should click the Improved Blasting Device to spawn a Blasting Cap. Do not DPS this either. Allow the tank to pick it up and keep it near the slime.
After a bit of time has passed, the Blasting Cap will self-destruct, splitting the Ichorous Ire into two. It's extremely important that the tank move out of self-destruct, as it can kill the tank as easily as anyone else. The tank should pick up both slimes and repeat the process.
This time, before the bomb can cast self-destruct, a Spriggan Quencher will spawn on the wooden platform and attempt to attack the Blasting Cap. The only role for DPS on this fight is to kill this Spriggan before it can kill the Blasting Cap. After a 3rd round of explosives, the Ichorous Ire will have become eight small slimes that can be killed normally.
Wake up the DPS, grab your loot, and head to the final boss.
This encounter will revolve around the Lv?? Boulders to the left and right side of the room. During the fight, Gyges himself will break the boulders on the right shortly after being engaged. While you cannot prevent this, you can pull him to the start of the room to delay it, forcing him to run further. Other than that, he only has one mechanic you need to be aware of: Colossal Slam. It is a heavy damage cone AoE that inflicts heavy damage. Face Gyges away and have the tank move out of Colossal Slam, and the healer shouldn't have a hard time.
Once Gyges breaks the first Lv?? Boulder, smaller gigas called Stone Servants will slowly spawn out of the hole and make their way to the Lv?? Boulder on the opposite side of the room. If you have strong DPS, you can ignore these and burn the boss down. However, I recommend one of the players inflict Heavy on it and focus on killing it before returning to the boss.
If the Stone Servants are successful at breaking the Lv?? Boulder, they will go after the player with the highest aggro, likely the healer. If Gyges is not almost dead by now, this can get extremely messy. Limit Break is highly recommended at this point to quickly end the fight.