I'm playing a ranger, and I don't have the able learner feat, so I haven't invested in spellcraft. I'm fighting against a certain wizard near the start of MotB, and he can drop me from 250+ hp to insta death

Here's the catch - I have a Scarab of greater protection that grants immunity to death magic. I can't tell what he's casting, I just do a save vs will, and if I fail, I outright die. 'Death Magic' spells listed on the wiki require fortitude saves, so all I can think of is 'Weird'

But Weird says it requires a fortitude save and a will save - I've looked through the logs three times now, and my PC only performs a will save - not a fortitude save (which I believe is the Weird portion that kills you) ((and I always die right after the will save DC in the log, as well))

So I'm totally at a loss - any pros know what spell this is, and why it can cut through an immunity to death magic and kill me instantly?

Try equipping an item that gives you immunity to mind-affecting spells. That will let you rule out weird (and phantasmal killer, while we're at it). If you lack such an item, the mind blank or protection from evil spells will suffice.

While there are quite a few instant death spells, most either require a fortitude save, can be blocked by death magic immunity, or can't instantly kill a character at 250 HP. Two possibilities I could find that could be taking you out are the epic spells Damnation and Entropic Husk. Both result in instant death if a will save is failed, and can't be blocked by protection from death magic. Note that both of these require a touch attack, so one option to avoid them is to stay out of melee range until the wizard fires them off at a different party member. Actually both Weird and Phantasmal Killer are bugged in NWN2 so that they can be blocked by death immunity, unless the OP has a mod installed that fixes this bug.

I found this excerpt in the wiki (I know it's just a wiki, but it's fairly accurate and well updated) under Phantasmal Killer

"Creatures immune to Death Magic (Death Ward, Deathless Frenzy) are immune to this spell. Phantasmal Killer shares this bug with Weird, which is also stopped by Death Magic. In NWN1, both spells bypassed death immunity, because the descriptor in both cases is "Fear, Mind-Affecting," and not "Death" (as in Slay Living/Finger of Death)."

And the lack of a fortitude save in the combat dialogue makes me doubt it is either spell... but these seem the only potential culprits, so I'll just have to assume it is one of them - that the fortitude save is unlisted in the dialogue but still occurs, and that the spells have been patched to bypass death magic

Getting some form of immunity to mind-affecting spells is a good idea before a mage battle anyways, so there's no harm in trying it. Still, after reading the explanation on the wiki it seems unlikely that this is the culprit.