Sounds like you need to focus more on building up your army in those first two weeks. Since they start you with a blacksmith, marketplace, and mage guild in your town, build a City Hall on day 1 to boost your income, then focus on getting lvl 1-5 creature dwellings (up through a cliff nest) built before the end of week 1, along with a castle and a mess hall. During week 2 get the lvl 1-5 dwellings upgraded. If you don't have enough money for all the available creatures then focus on Thunderbirds, Wolf Raiders, and Hobgoblins (in that order), as they're all fast, hit hard, and benefit from Tarnum's specialization of Offense. Be sure to also grab all the free goblins from the nearby dwellings. This should give the most powerful army you're capable of getting within the first two weeks, so after that it's just a matter of crushing whatever hero the enemy sends at you.
Hopefully you loaded Tarnum up with a good set of secondary skills during the first mission, along with all available primary skill increases and every spell he was capable of learning. In these campaigns building up Tarnum quickly in the early missions is very important, as it can easily make the difference between the later missions being very difficult or a complete comp stomp.
