Let's see, we only need to consider the prepared spells that actually require saving throws, which are:
Wizard Spells
1st—cause fear (DC 14)[Necromancy], charm person (DC 14)[Enchantment];
2nd—blindness/deafness (DC 15)[Necromancy], glitterdust (DC 15)[Conjuration];
4th—ice storm (DC 17)[Evocation].
Cleric Spells
1st—bane (DC 14)[Enchantment], sanctuary (DC 14)[Abjuration];
2nd—enthrall (DC 15)[Enchantment], hold person (DC 15)[Enchantment], sound burst (DC 15)[Evocation];
4th—wormtongueᴰᴺ (DC 17)[Transmutation];
5th—curse of the wormsᴰᴺ (DC 18)[Conjuration].
Which makes:
Spell Schools (spell level = DC–13)
Abjuration—sanctuary (DC 14).
Conjuration—curse of the worms (DC 18), glitterdust (DC 15).
Enchantment—bane (DC 14), charm person (DC 14), enthrall (DC 15), hold person (DC 15).
Evocation—ice storm (DC 17), sound burst (DC 15).
Necromancy—blindness/deafness (DC 15), cause fear (DC 14).
Transmutation—wormtongue (DC 17).
So it looks like Enchantment is our best pick for Spell Focus with maybe Conjuration as a back up.
How about Augment Summoning, Greater Spell Focus (Enchantment), Scribe Scrollᴮ, Spell Focus (Conjuration, Enchantment), Spell Penetrationᴮ including his bonus feats?