Let's max out Concentration, Know (religion), Know (the planes), Listen, Spellcraft, Spot. Sound good?
That'd give it:
Skills: Concentration +20, Knowledge (religion) +21, Knowledge (the planes) +21, Listen +20, Spellcraft +21 (
+23 decyphering scrolls), Spot +20
Hmm… I'm tempted to give it some ranks in Use Magic Device as well.
Maybe trim down the Knowledge skills to 11 ranks to pay for it?
Let's see, that'd make it…
Concentration 17 ranks, Knowledge (religion) 11 ranks, Knowledge (the planes) 11 ranks, Listen 17 ranks, Spellcraft 17 ranks, Spot 17 ranks, Use Magic Device 12 ranks.
Skills #2: Concentration +20, Knowledge (religion) +15, Knowledge (the planes) +15, Listen +20, Spellcraft +21 (
+23 decyphering scrolls), Spot +20, Use Magic Device +15 (
+17 with scrolls)
Which do you prefer?
For feats, how about some Spell Focus and Spell Penetration, maybe metamagic?
We could give it Spell Focus several times since there's a wide selection of harmful save-required spells in the Priest's prepared list:
Enchantment—
bane,
bolts of bedevilmentᴰ,
greater command,
enthrall,
lesser confusionᴰ,
touch of madnessᴰ
Necromancy—
cause fear,
death knell,
poison,
slay living
Illusion—
silence
Evocation—
flame strike,
sound burst,
unholy blightᴰ
That's six Enchantment spells, four Necromancy, three Evocation and one Illusion.
So how about Spell Focus in Enchantment and Necromancy plus Spell Penetration and a Metamagic feat.
For the Metamagic how about Heighten Spell since it'll let them Chant Cast spells with augmented save DCs?
That leaves one feat and I fancy spending it on something combat related rather than more magic boosts.
Perhaps Improved Critical (tentacles) or Combat Reflexes?