Go to Rox and Braham, wait for the mobs to spawn, kill them. After killing them, proceed into the tunnel, there are 5 caves in the tunnel, each having a group of mob in it, after killing the mobs, destroy the Boulder that becomes damageable (a red gear indicates it's place). After killing all 5 group of mobs and destroying all 5 Boulders on the red gears you wait for the Drill to make it's way out of the tunnel (which should be already done).
You exit the tunnel and kill the group of mobs coming to you. Then proceed to the Steam Walls. The first Steam Wall is unpassable, each Steam Wall knocks you back and damage you. After the first, you can pass the following Steam Walls by walking backwards in them (do not walk into them with low HP, that downs or even kills you) or use Stability. Make your way on to the weapon testing arena.
You enter the arena and survive the 3 weapon test. After entering the arena 2 Champions spawn, separate them and kill them. Not separating them results in Resolution being applied. The weapon test types are in random order:
You have to dodge the Fire Tornadoes coming towards you. The Tornadoes apply Burning and knock back. Please note that since the rework, the Fire Tornadoes go through the Thermal Core. After surviving the test you can damage 25% of the Thermal Core.Fireball Barrage
You have to avoid, block or reflect the Fire Barrage falling on you, place reflect (Guardian with Wall of Reflection ) or absorb (Revenant Legendary Centaur Stance with Protective Solace) in the middle of the AoE to absorb the projectile. The AoE after the projectile landed applies Burning. After surviving the test you can damage 25% of the Thermal Core.
You have to dodge, block or jump over the shockwaves coming towards you. The shockwaves apply Knockdown. After surviving the test you can damage 25% of the Thermal Core.At 25% and the boss after
You have to damage the core as fast as possible since now all 3 weapon test types are active. After you killed the Thermal Core the Weapons Test Engineer and it's two champion comes down. Separate them and kill the champions on the sides. The Weapons Test Engineer gets Invulnerability every 25%, does an AoE 10 stack Bleeding and moves to one of the 4 spots which is the middle of the 4 walls. (It can stay where it was). It is random where he goes next. Kill him to finish the fractal.