This is a pretty spot on strategy, I have to admit! I was just explaining essentially the same thing to someone else the other day...use Isaiah to absorb Queensguard hits and just pile on the damage to prevent her from attacking...
In addition to the "Deteriorate" spell, the "Maalik" revolver and "Shepherd" 1-handed spear can cast "Consecrate", which is another 5% per turn automatically (though it can miss)