From the slug room follow the path right down. There's some stuff to collect where the path breaks right further. Take it, come back and continue to the room with some barriers (#1). Before breaking them, head on straight to a chamber with a couple of dwarves for another trunk of gold (#2). Send your minions to bring it to the waypoint and head back to the barriers room (#1).
Break the barriers and kill the dwarves. You'll find next trunk of gold and a wheel in the back of the room (#1). Take the gold first. The wheel opens passage to the following mine areas - come back and open it when you're done with the gold. Kill the dwarves which will come out. There are some barrels and some loot there on your right. Further left you'll come to red spawning pit and you'll meet more dwarves (#2). Kill them.
Head up the stairs (#1) to the wooden platform and you'll see some elves imprisoned. Kill the slugs (left) and cover their hole with the cart. Turn the wheel (#2) and let the elves go. Take the gold and break the barriers to carry it to the waypoint. Send minions to open the next pass with another mechanism - it'll lead you back to the room where you killed the first slug. Carry the gold to the waypoint and let's get out of here!