One that people tend to really overlook is Owl's Insight. It's an insight bonus to Wisdom (so it stacks with enhancement bonuses.) It's equal to half your caster level (so things that boost CL can get it to a really impressive total.) It lasts for an hour (plenty of time for the big fight, especially if extended.)
Even at the level where you get it, with no juicing of caster level, it amounts to a +4 to Wisdom, and thus a +2 to DC. Not bad for a fifth-level spell, and that's the worst-case scenario.