A newbie asks...

[thejeff]

That's really only necessary for the toughest of Pretenders. Just get a look at what he's got - send in a scout or commander set to retreat, then figure out how to take him.
In this case it sounds like magic weapons were enough.

Otherwise see what elemental resistances he's missing and script single target long range spells against those. Or spam MR spells at him, the AI rarely boosts MR enough. Enough Soul Slays will kill even a 20+ god.