Holy martial strike, an epic feat for anyone with smite evil, or a class feat for Fist of Raziel, is the ticket.
It causes every melee weapon the character carries to do 2d6 divine damage to evil opponents every time to hits. Someone with maximum BAB (four attacks per round, or eight if hasted), can do formidable amounts of damage rather quickly, even to a demilich). Even better is a dual wielder (both weapons get the divine damage).
Make sure to give the character(s) as much protection as you can, as demiliches can cast a lot of spells at will.