Start walking across the path that leads across the lava fields. Try and stay near the centre of the path as you fight the Skaarj you encounter so you don’t fall into the lava, at the end of the path cross a rope bridge into the Sun Spire.
A Skaarj will drop through the ceiling, after killing it proceed ahead into a chamber labelled "Primary Ascension". Ride the lift in the centre all the way to the top, kill a Tentacle and the Gasbags, go through a door upstairs labelled "Cistern", and kill the monsters beyond. Jump into the Cistern water and swim into an underwater tunnel which leads back above water where you’ll find a sliding panel leading to a dark room with a water trough. Kill the monsters here and find the left corridor of two side by side exits using your dispersion pistol or flares for light. This leads to a small dark room, kill everything there and head through the room to a T-junction with a wall inscription of "Great Hall". Take the leftmost path until you emerge on a balcony overlooking the Great Hall itself.
Kill the enemy on the balcony and collect the flares then drop down onto the main floor. Press a lever on a darkened wall to raise a podium in the centre of the room and lower a lift. The podium contains a MiniGun in a hidden compartment.
Facing the large banner on the wall, take the leftmost corridor out of the Great Hall until you reach the lift. Keep taking lifts up, killing any enemy you encounter until you reach the top. Now take a corridor leading away till you find the message "Chamber of Crystals", leave this for the moment and proceed down the hall. Kill the Skaarj you meet and explore the roof of the Sun Spire for gear. Return to the Chamber of Crystals and throw the lever on the floor.
Run back to the roof and climb a short staircase to a spot with the inscription "Prepare to embark." Wait here until a lift drops down, get in and wait for the lift to transport you to the next level.