Do you have access to the metamagic Feat Dazing Spell? Slap it on anything that doesn't target Fortitude.
Other than that, Grease, Glitterdust, Stumble Gap... anything that targets Reflex or Will and applies status modifiers wil make things much easier.
If you know Giants are coming, a couple Charm Person / Hold Person spells might even be worth the two slots they take memorizing.
Don't bother relying on damage spells. The one thing Giants bring to the field is a crapton of hit points, coupled with high Fortitude Saves.
Don't neglect your defense. They'll be able to hit your AC if they get in striking range or throw rocks, so miss chance is the way to go. Mirror Image is your friend. Improved Invisibility is even better.