Try to modify his behavior. Phantom a stack and keep it BEHIND one of your own stacks. This gives Baal a greater chance that he will smash the two stacks that are one behind another.
Whenever he summons, do not kill the imps. They are no-retaliation units, so leave them be. There is another counter which checks how many allied troops the boss has. The more they are, the more the counter decreases.
Now, since I'm referring to the counter all the time, here's its "weight":
If there are 2 troops that the boss can hit with his sword - 220
If there is one troop that the boss can hit with his sword - 20
Flying sword - 75
Summon demons - only if there are less than 7 boss-allied troops. with each allied troop the weight reduces by 10. Basic weight is 100, so it ranges from 100 (0 allied troops) to 40 (6 allied troops).
So in general, the boss is most likely to:
1. Hack 2 troops one behind another.
2. Summon demons.
3. Throw his sword.
4. Hit a single troop.
Try to use this to your advantage
I'm not saying anything about stone skin, as you're obviously using it on all your troops. The next thing, leave most of bill gilbert's troops to die, but keep his knights with stone skin all the time. They are very important, as they can survive the longest without overburdening you and your number of casts. When you have free, summon paladin phantoms and ress your main paladin stack - he's the most important, as he is the one that will reduce the casualties after the boss is down.