This is the giant mission. Lord Sigmund and his giants love lava, apparently. Half the trick (maybe 90% of the trick) of this level is to click quickly. You have to build a ton of bridges where you build the bridge, get the imp to cross it and start taking over territory before the bridge burns up. Then you have to get the imp back onto normal ground. Use the salamanders for fighting since they can get back home across the lava without help.

Dungeon Keeper 2 Walkthough Index