To earn this achievement you must complete the second confession level - Resentment. To enter the mountain you must obtain a trophy from one of the lair bosses in a prior region.
The final boss for this level isn't that bad imo, probably easier than the first confession boss. I beat it on my first try with a sub-optimal team. You're going to need to be able to comfortably deal damage to all 4 enemy locations and mitigate AOE attacks on your own team. The main gimmick of this boss (a dragon) is that it has two lungs on either side of it (in the first and fourth positions). At the start of each turn one of the lungs will take a shallow breath. If you don't interrupt the filled lung by the end of the turn the dragon will unleash a powerful AOE attack (Sundering Exhalation) that hits your entire team. This attack can crit for 30+ damage so do everything in your power to avoid it.
To disrupt this, you need to deal 12 damage to the first lung or 9 damage to the second lung, whichever is filled. Occasionally the dragon will take a deep breath which will cause both lungs to fill and you will have to interrupt both before the end of the turn. Unfortunately, I don't know what exactly triggers this as it only did this to me once. If you kill a lung it can no longer take a breath in that spot, but killing lungs is not necessary to win the battle. Once the dragon is dead the fight is over.
I would recommend a team that can put DOT on the dragon to give you the flexibility you need to reliably interrupt the lungs. If you are able to block the majority of Sundering Exhalations you should be fine as the rest of the dragon's attacks are relatively week. The lungs have 200% resistance to bleed, blight, and burn so don't waste your time trying to DOT them. I beat this with a super buffed Flagellant who was tanking almost all the damage for me via More! MORE! while the rest of my team did the damage. I had a Hellion up front to Iron Swan the second lung, but this was not reliable and not something I would recommend if you are specifically planning around this boss. Something like a Highwayman or Grave Robber with more flexible range should work much better. In the back I had a Jester and Plague Doctor that rotated positions to debuff enemies, bleed the dragon, manage stress, and heal my team.
The only time it can get scary is if you fail to interrupt 2-3 Sundering Exhalations in a row and your entire team winds up near Death's Door. Try to come into the fight with full health, healing combat items, and multiple options to heal or mitigate damage within your team and you should do just fine.