In order to reach the main chamber in the SV, you have to first kill both of the lesser bosses. Hydromancer Thespia has two water elementals and uses storms to attack and Mekgineer Steamrigger is a mechanical similar to Thermoplugg's pounder. At about 75%, 50%, and 25% health, Steamrigger will call gnomes to repair (heal) him. He also has various special attacks that do not do tremendous amounts of damage.
The final boss (after the doors are opened) is Warlord Kalithresh.
There are access panels behind each boss. Click on them to open the door.