These areas have ca. the same difficulty. They want you to start thinking what you play out, and start designing half-solid decks.
The Heroes start to have some life, so the battles will actually take time, so a little life on your creatures will definitely help your case.
You'll probably did not have the chance yet to buy a lot of new cards, but you'll probably at this point start to be familiar with the Challenge Hall, and get some equipment (weapon, helm, armour) from it.
Do not worry too much on what cards you have avaiable. The cards you get from the story-mode (Map-battles) and what you find at other places will most probably be majorly Human and Elf cards, and maybe some other races for flavour.
Your deck should mostly be assambled from 2 turn charged ones, by type including cavalry (charger and pegasi), infantry and archery (preferably snipers for more damage). But whatever works for you.
Probably this is the point where you'll start to get a hint of your class' usefullness too.