I spent 2 days fighting Impossible Baal using my mage with poor equipments, even no "Turn time back" spell in the whole game.
I tried many types of troop, as last I chose Lizardman army.
Main Tank:Brontus
His job is attacking baal and getting hit. High defence, high HP,even a spiked skin make it a great tank.Adding avenger angel can be more powerful.
With Lv3 Stone skin, Divne shield, plus mage shield, can minimize baal's damage to 500 or less.
Main Dder:Gorguls
Two blocks attack, and re-action after killing an enemy(plus frenzy) make it a great dder in Baal fight.
Distant Dder:Gorguanas
Protector: Arch Mage
Supporter: Paladin
Important Equipments:
Singing Dagger: +5 attack +15% lizardman attack
Claws of K'Tahu: +7 attack +1 initiative and morale of lizardman
Scaly bracelet: +10 mana +2 morale of lizardman
Pilgram's Boots: +1 speed to slowest troop
I killed baal in 10 rounds, then spent 2 round recovering armies.
Here is my screenshot of this game, the 1st is a failure, Gilbert's troops killed baal's minion when I was recovering armies.
Mage lv 53 ,spent 61 days
No lose , Damage dealed 90% by troops
I'm using Red Dragon-Oil-Phantom-Burning tactics when fighting most of enemies.