- Invade Lord Darius's Keep and slay him
- Build a Library and use the Warlocks to research new spells
- Train up your creatures in the Training Room
Much like in Warcry, all you really have to do is follow the Mentor's instructions. Make sure your Library is at least 3x4, as you will have two spells to research and upgrade, and one potential magical item (a Mana Boost) to find. Your Training Room will need to be at least 4x4 to accommodate all the creatures you will want to train.
It may be prudent to train your Warlocks as much as possible before letting them research Create Imp and Thunderbolt, as the latter seems to be what triggers a breach and incursion of Dwarfs into your dungeon. After dealing with them, though, you will have more than enough time and gold available to see all of your creatures trained up to level 4 if you wish (although the Mentor will urge you to break in and assault Darius's Dwarf sentries long before then).
If you want to try to claim the Mana Boost to the southeast of the Keep before killing Darius (so that it counts on the Debriefing Screen), you will have to lure out the Dwarfs with clever use of creature-grabbing and -dropping. Either way, Darius and his bodyguards should be no match for your well-trained forces. Two Portal gems down, 18 to go!