> How many hit points/spell points do I need, realistically, to take on the Grim Reaper? One of my characters is just under fifteen hundred HP, but the others are an average of 600.

Assuming 600 as your lowest HP, I would say that you can win against the GR, but that's with a well-equipped party and continuous healing via a spellcaster or artifacts. You should boost your armor for the entire party to 60 to avoid parylization and damage from physical attacks, and if possible use a silence spell to safeguard against spells cast against you. This leaves elemental attacks, which you can't really do anything about once you're in battle, so try to take out the elemental breathers first.

Whenever I fight the GR, I always take out all his companions first. If you can survive long enough to get him alone, then you can usually defeat him.