The trouble I have with Spell Penetration is that the Spell Penetration feat only works to overcome SR, so if he doesn't have SR....
So I much prefer the spell turning based option, as Cleon roughed out above:
Reflect Magic (Su): Any spell or supernatural attack that specifically targets Ythog-Nthlei is reflected back onto its caster as per the spell turning spell.
Well I'd be OK with
Reflect Spells or
Reflect Magic if you don't think they're too nasty. I'd lean towards the Reflect Spells version to leave the field open to supernatural attacks.
I'm not sure what's the difficulty if the "Magic Reflection" writeup says the Spell Penetration feat applies to the check?
I guess if it bother Freyar, we could rearrange it to something like:
Special Qualities: Spell reflection, spell resistance X (targeted spells only)
Spell Reflection (Su): Any spell attack that fails to penetrate Ythog-Nthlei spell resistance is reflected back onto its caster as per the
spell turning spell.
Spell Resistance (Su): Ythog-Nthlei's spell resistance is only effective against spell that specifically target Ythog-Nthlei. It is useless against area-effect spells such bursts or spreads.
Although I'd be tempted to tweak the spell resistance further to:
Spell Resistance 2 (Su): Ythog-Nthlei's spell resistance is effective against any spell that targets Ythog-Nthlei, even if the spell normally does not allow spell resistance. It is useless against area-effect spells such bursts or spreads.
We could save the above for that lower-CR "Aspect of Ythog-Nthlei" we were talking about earlier, I suppose.
So which are we going for?
Reflect Spell is the closest to the original monster's ability.