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.
I say use spell resistance 2 makes for an even more unique creaturewhile still giving people a loophole to attack him.
The difficulty with the Magic Reflection is adding ticky-tack little things to Spell Penetration. Just seems messy.
I guess I'd probably prefer Reflect Spell as opposed to any of the exceptions listed above, especially since it's closest to the original ability.
Did we have more on the mace? If not, shall we work that out now?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.