Am still stuck on the 2nd campaign mission.
Game is tough. Get so tired of failing over and over again.
I had a tough time with that one, I also putzed around researching and building while the enemy keeps getting stronger. They kept sending single guys to cap my towns, and I couldn't dump out enough troops to keep them occupied long enough to keep them. When I fortify my position they'd send stack of hero + tier 2-3 units that just run over my sad defense.
Eventually what I did was just build stacks of flamepot throwers and a shaman, and just kept it at choke point towns. That keeps the single cappers away long enough for me to build up the army and roll through a few towns. They're cheap and build fast, you can crank out a stack of 5 + shaman really easily. 2 fulls tacks will easily defend your town.
What kept me afloat offensively (and defensively on the odd occasion) was summon beast and summon demigod (or whatever that giant thing is). The beasts you can summon are ridiculously good (spider queens, electric serpent, etc) and they will be more than a match for even tier 3 units. The beasts have cheap upkeep and the summon casts quickly.
The flame throwers are really good on town defense, especially if you have some walls. they absolutely wreck tier 1 and 2s and with a shaman you can heal them up slightly as long as they don't have flying. They also rank up quickly. However they will get mauled in the open field and they will get demolished by tier 3s.
In the end I ended up using 2 hero stacks with some summoned giant things (3 of them) and some fliers and just demolished a few towns. You definitely need to keep 2-3 stacks back to recap stuff that gets taken though.