Baal on hard mode as a mage

[Zechnophobe]

Try using a unit that summons things. Dryads for instance. Phantom them at some point, and use the phantom + thorns to try and force 'two stack attack' from him on bill's forces. Using real units like Bone dragons or black knights isn't efficient for the most part.

I'd also consider not using Archmages. As much as the shield is nice, their lack of damage isn't.

Mass SPells effect Bill too, so casting Mass Demonslayer can really help take down 'walls' of demons pretty quick.

Hope that helps!