First help Grom and his people destroy all the human buildings and you will get the above listed objective. Which is simple enough eh? Build this like a human base with the great hall near the gold mine. Set up some defensive structures around the border of the area you want to control. Also, keep your burrows near where your peasants are working so they can escape inside should any enemy threaten them. They'll then fire out of the windows. After you've got your great hall constructed you receive new mission objectives: Secure the area around the laboratory and bring 2 zeppelins to your base. Before too long, maybe before you get your first military unit produced, the impatient Grom will attack the human base. Let him deal with his own problems. You'll be given the optional quest for this chapter once that event occurs:

Optional Quest: Pacify the Humans
This should be one you'll want to do as long as you've gone a while without some good action. The first base I recommend you attack is northwest of your base. Not the blue base that grom attacked, but a cyan one. You can follow the mountain west of your base as it curves to the north, then ends and allows you to move west. Use some of grom's spirit wolves to find it if you like. You'll need to build up some forces first, though. I also recommend you upgrade your units' armor and weapons as well.

I took Thrall, two kodo beasts, six grunts and three headhunters in one group and several raiders in another. Notice that raiders have siege type damage. Which means they can do some good damage to structures. I prefer to use massed assaults to destroy buildings as it does net you some resources. Honestly, I only lost two grunts and a raider attacking this base. Northwest of here there are a group of centaurs you can kill. One of them drops a tome of intelligence and a potion of greater mana. Definitely use the tome.

Just east of there ,around the mountain (just south of the blue human base) are a group of quillboars. They don't drop anything, but a crate nearby has a potion of healing inside. After you replenish your forces you should attack the grey base. If Grom's attacks didn't reveal the grey base it's directly north of his base. This should be no more difficult than the cyan base except there is a cannon tower here. This is one of the reasons I like raiders. They're quick and they can do some nice damage to buildings. Use your raiders to take out the cannon tower while your grunts, headhunters and Thrall take care of the human forces. If you have the spirit wolves helping you and use chain lightning frequently the forces here shouldn't take too long to be destroyed.

Once that's done I prefer to keep my headhunters and thrall out of the battles. The raiders and grunts have the pillage ability and will net you some nice gold for your trouble. The trolls and Thrall just do extra damage to the building but don't get you anything out of it. When you're finished pillaging the grey base, move on to the green one with some replenished forces. It's to the north of the grey base and is no more challenging than that.

Before you enter the green base note how the path branches off to the northwest. Take that path and you can fight some powerful owlbears. The biggest one drops some claws of attack +9 for Thrall. If you don't have room simply select your weakest item and replace it with the claws (if you like). Move on to the green base and destroy everything there. Now that the enemy is almost completely pacified you can take the rest of your forces that were guarding your base to eliminate the final (blue) human base, then move on to the northwest and fight the centaurs (the first group drops a potion of healing, the second drops gauntlets of ogre strength +3 and a scroll of healing.

Finally, right next to the laboratory you'll encounter a group of harpies. They're pretty powerful so be careful and use your raiders to ensnare the big one. When she dies she'll drop a circlet of nobility (+2 to all stats). At last you can purchase the two zeppelins and fly them to your base, completing the mission. One other thing: I froze my gold and wood at 1162/3161 and checked the difference when I assaulted the blue base. When the raid was over I had 1968/3528. Not bad for free resources, huh? I think I may never use catapaults unless the enemy has some very strong fortifications.