Edit: It appears it was indeed a spell. I thought otherwise because my characters were low level anyway, and the spell damages as well as causes eradicated. The Demon Lord always targets my Paladin with the spell, and this time he had around 30hp remaining and was eradicated. So I too feel that Protection from Magic(Sorcerer 6,4) should be best. It provides around 30% protection from magic from what I've seen on average for me but the amount of protection depends on level. Also, one can hope that the temple blessing and protection from magic stack, and then go in to beat him. Of course I do not know if the Demon Lord respawns or if he's done after you beat him.

Original post: Eradication is not a spell- wiki and other places say when damage is too great, dead or unconscious people will get the eradicated status instead(or even good condition people will go directly to eradicated instead of unconscious or dead). That attack probably has an unholy amount of damage- Portsmith stuff is the last town to be handled anyway- even if it is marked as town 2. Go by the main quest(you'll get the quest in the cavern below sorpigsl if you haven't already, an old man will ask you to deliver a scroll to someone. The order of the town's you visit on that quest is the main order of towns). Your party is too weak, if you're not at the end. I've personally tried these battles again and again and again at lower levels- it is a matter of possibility, there is none. The world is large, you'll have to do other stuff till then. Main quest, side quests, random roaming around, it's up to you. You don't have to follow everything in the game- this game has two layers. The first layer is for players who play blindly, in this case levelling up and visiting springs that offer permanent boosts surely help most. You'll come across those. The second is for those who use the maps in the cluebook, which highlights all the secret areas and want to get done fast. This is a different game altigether- the super secret areas with convoluted placing are so that these players may get another thrill- there's lots of obstacles in claiming these secrets. The benefits are not necessary to win the game. The nature of the benefits may make them useless too to players who permanently increase stats by levelling up, equipping gear, finding these springs, discovering regions like structures and castles, caverns and dungeons. In time, the demon lord fight might as well be a breeze- with the right spells. In among battles, specialist equipment also matters, equipping certain weapons and armor for certain fights can give an advantage- though I'm not sure there's any for the demon lord, I haven't found anything like it. Examples of these gear are fire and cold shields when facing those enemies(like cold shield vs white dragons might help), and this thunder hammer I found for cleric class, which can be equipped in areas where magic cannot be used- at least the damage bonus will be useful.