The third challenge-area it is.
The enemy here are Ogres. Basic Ogres, and the worse type: Enraged Ogres (which gain attack as the get damaged).
Two special boss-ogre also comes at Stage 2: the green aura-one is way more easy and don't require special attention. On the other hand Ragnar is a real trouble with its 24 HP and enragement, and the extra attack it provides (has blue aura).
The special thing is on Stage 2: some walls on the side of the AI. It is usually considered Bad Behaviour attacking these walls, as they hinder the ogres a lot, narrowing them for quite a time to only two rows. So putting down things with ability of Vigilance, Whirlwind or Cone of Fire (etc.) requires carefull attention, and trying to build up heroic-thingies by destroying the walls is a totally bad idea.
Stage 2 works with 2 bosses, the upper one holding massive direct-damage cards, the bottom-one having all the creatures. It is practically impossible to kill them before they run out of cards, so just be patient and let them bleed to death.
The AI has a buff what worth mention: it gives a lot of attack for the price of Immolation (= self-damage for the duration). It worth considering how far a (lone) ogre can reach before spaming cards trying to destroy it, as it might just die on the way without reaching anything. Knockback-abilities can add to this tactic.