I don't know if this is a bug or not, but we're stealing
nothing if Carsomyr is unable to dispel with Spell Immunity.
Dispel is always abjuration school, so Spell Immunity should
really block it.
Carsomyr doesn't work vs high level enchantments? Makes
sense though, it's an EVIL-smiter weapon, not a MAGE-smiter.
Mages have the edge in magery....
The real bug IMHO is in the arrows!!!!
Anyway, you keep changin his equipment even resorting to items
which come in
LIMITED supply...

Unfair...
While I have to change the choice or order of my spells!
So I say, I'd go for MindFlyer's Collar and have him dominated!
I won! Go me!
So, it seems like his abilities are not enough!!!
And even with the arrows, he wouldn't have an easy ride...
I'd go concealed as usual... Once buffed, melee... He could
use the bow at close range so I can hit better for a while.
Assuming the arrows work (I think there's always a check...),
I can always quick-cast some spells again, not sure who
would be the winner...
Actually, and surprising, really there's only a thing that he could
try: War Cry!!!
(Though I hardly think Lofferul took this one...)
A clean Save vs Spell.
My Kensai/Mage lvl 12/14 would have a to score 4, considering 25% MR (equipment), we have a resulting 85% chance of saving...
Still greatly against his odds, and this is already a defeat for Keldy,
having to rely on the crappiest ability, hoping for an unlikely
result... and given the fact that I'd go concelead in the beginning,
maybe he couldn't even chase me!!!
Or following your UNFAIR steps, I could say: "Let's take a Kensai/Mage
with the long sword DragonSlayer, so he's immune to fear!!!!!!!!!"
NO!!! Let's just put in the starting sequencer: Invisibility, Non Detection,
Emotion!!!!! We're immune!!!!
Go me!!!
ehehehehehehehehehehehehehehehe
Anyway, let's collect the results.
Loffelur vastly underestimated mages, but maybe we (I) underestimated
a bit Keldy, as an Inquisitor is the only warrior type who can
provide a challenge. Yet we've demonstrated that a dual has LOTS
of choices and abilities to win in all cases, while he can only
hope in alternative equipment or very unlikely events.
Surely with the originally listed abilities he was in VERY weak position...
And we're still playing with a great gap in levels, just to remember...