I've been asked about Baal's stats, so figured it'd be best to answer here rather than somewhere private. Baal's resistances are based on monstats.txt, skills.txt (his salvation aura), and UniqueItems.txt (he has an inventory, just like Valkyrie and Assassin Shadows).
Monstats.txt ResistancesCode:
Phys Mag Fire Ltng Cold Pois
Norm 30 25 60 60 60 60
NM 50 40 75 75 75 75
Hell 85 75 99 99 99 99
skills.txtBaalSalvation = Random +25-40% for each resistance separately, randomizing every 3 seconds
UniqueItems.txtNote that I don't have access to monequip.txt, so I don't actually know what baal's wearing entirely, but it's probably just this hat.
BaalBoss (cap) = +95%PLReduced, +55%AbsorbEle, +35%AbsorbMag +40%PhysRes
Since Baal's base resistances aren't higher than 100%, I'm told that Conviction, LRes, etc will have full effect against breaking his immunities, rather than acting at 1/5th effect. For example, in Norm Baal's phys res will be 30% + 40% + 25-40% ( = 95-110%), but an AmplifyDamage of 11% should break his immunity in the worst case. It's unfortunate that there is no -%PhysRes gear. I have not experimented with this or heard of reason to doubt it. It looks like elemental damage with loads of -%EleRes gear and a powerful Conviction and LRes is the way to go in Hell difficulty.
<edit> Corrected Norm PhysRes range from 95-135% to 95-110% </edit>