Yes, this game really is that brutal, and it's only going to get worse. Be glad these enemies get only one attack per round, and that the attacks only have a 5% chance of instant kill. Trust me, it gets worse.

Anyway, to cure death, you can use the following:
Rizefus, 6th level cleric spell: Restores 50% if successful, usable both in and out of combat.
Rezefeid, 7th level cleric spell: Restores 100& if successful, usable only out of combat for some reason. Does not cure Ashes in Elminage Gothic (I believe it can in other Elminage games).
Miracle: 7th level mage spell: Usable only during combat, and drains one level from the caster, but one of the possible choices is to revive and fully heal the party; this *can* cure Ashes. If the option does not appear, you can just cancel out of the selection and try again a different combat round. (Note that this is unlike Wizardry's MAHAMAN spell, where you can't back out of the spell once the choices are made available.)

I could point out that clerics and mages get all their spell levels rather early (level 13 for 7th level spells); you could easily expect to reach level 30+ by late game, and the final bonus dungeon can easily take you up to level 400 or so (and is still going to be brutal).

One more thing: There is actually a penalty for reloading a mid-dungeon save; time will pass, and if too much time passes, your characters will get older. Keep in mind that reviving will age the character (regardless of method, as far as I can tell); 1 year for death, 2 years for ashes.

To guard against instant death, your two options are:
1. Beheading resistance will guard against all forms of instant death, but getting it to 100% is quite difficult.
2. If you have 100% or more Dark resistance, instant death spells will not affect you at all.

By the way, instant death spells are useful for the player, especially if you check enemy resistances in the bestiary.