Torchlight 2 Walkthrough Part 19: Some Assembly Required Part 2

After completing The Cave In and blowing up the pass, you can continue with the Some Assembly Required mission, which was your real objective in the first place. Head through the pass and into the Middenmine area. Here, venomous and rotting creatures rule the day. You will want to equip items that give you resistance to poison if possible. Also note that ice attacks deal bonus damage to many of the enemies you'll encounter here. As usual, proceed through the randomly generated dungeon to search for your waypoint. The majority of this mission will actually be complete by now, so it's just on you to search the area for named enemies and treasure.

Once you have sufficiently uncovered every corner of this area, head towards your waypoint for the area's boss encounter. The boss here is the Bloatfang, a giant fly that patrols the dank corridors of the Middenmine. This creature will summon corpses to fight alongside it. Stay away from the corpses lest you get overwhelmed. Use area of effect attacks for crowd control if you can. If not, try to funnel enemies into as close to a single file line as possible before wearing them down with melee attacks.

When you are ready to take on the Bloatfang itself, you'll find a surprisingly easy fight awaiting you. The Bloatfang doesn't take too much in the way of hits to put down. Simply keep up a concentrated stream of attacks to the boss and he should go down without too much of a fight. Once the creature falls, collect the Automaton Power Source item and head back to the Imperial Camp to proceed with the story.