Pillar Of Flame and others spells like it hit more then one army on a tile. Now that the AI attacks and what not, it tends to move its armies around in a big stack. I was able to hold back the Gilden advance by casting Pillar of Flame once to kill all 3 of his armies. Also tested Tremor, Freeze, and Wither they also work on every army that in on the same tile.
AI has been awesome so far. Playing on normal for my first game, with 6 AI. I killed 2 of them so far and a 3rd died to the others. The last 3 AI players are puting up a fight. One even took my capital from me with my sov and his army right next to it, they just ran right by and took it. So on the next turn i had to fight that army and the city defenders to get back my capital. The AI is even using magic when they have a champion.
Maybe just a cooldown of about the same time it takes to train a pioneer. Other then that i don't see any need to change it. its a perk for playing the Pariden after all