Buy Pearls of Power so you can recover some spells.
Cold Ice Strike is a fantastic level 6 spell. Swift action to wreck anything in a 30' line is always quite useful to my cleric.
Holy Word at level 7 can cause some serious havoc on minions, and possibly even the boss, depending on their level.
Waves of Ecstasy isn't a bad way to stagger a bunch of minions either.
I often have a Word of Recall prepared, just in case things go really poorly and we need to escape.
If you have time before the day of combat, maybe grab a Planar Ally to help?
If you're the only spellcaster who will be casting beneficial spells, having a Spell Resistance prepared for use against a spellcaster boss can be a life saver.
I've gotten decent mileage out of Greater Command for locking down large groups of minions.