Well, obviously as you stated there are the spell focus feats for necromancy, plus pumping whatever your primary stat is. Assuming you're going with the bestow curse spells, pick the best spell possible to you and use it with spell perfection. It requires metamagic feats which is fine. Ones I would suggest for the curse spells are persistent, reach and maybe echoing or even heighten (although heighten is so-so).
Other than that I'm unsure.
Increasing the DC is likely done through debuffing your opponent rather, a witch's hexes might be very effective, possibly you might gestalt with anti-paladin too, they have some good debuff potential with their touch of corruption and aura giving a significant penalty on saves too.
Oracles have some specific spells for cursing too and are rather thematic as well.
Cleric with the luck-curse domain. Give them a -2 penalty to my saves in round 1, curse in round 2.
There are a few traits that boost the save DC of a specific spell.
My cleric build does the following:
Spell Focus Necro, Greater Spell Focus Necro, Heighten spell (to increase the spell DC by raising the level of the spell).
Active spell: Aura of Doom (if subject to fear there is a good chance the creature will be shaken for a -2 penalty to saves).
Round 1: Malign Eye the target (-2 penalty vs my spells)
Round 2: Heightened Major Curse the target, use a Rod of Persistent spell to force the target to roll twice and take the lowest roll.
Enjoy :)
- Gauss