Pielorinho said:Druids, if they're lucky, will have a warp wood spell available. Giants often fight with weapons with wooden hafts; destroying these weapons will definitely help.
Ridley's Cohort said:I advise caution with Improved Invisibility.
Giants can do a lot of damage. If too many PCs go invisible for a tough combat, those that remain visible draw more attacks each round. That can result in sudden PC deaths.
Sometimes it is best to coax the opposition into spreading the love around.
I would encourage the Rogue to use Blink, Mirror Image, Blur, etc. to boost their defenses instead.
An alternative tactic is to team up with the Cleric. Holy Smite or Order's Wrath (Reflex save or blind for 1 round), and Sound Burst (Will save or stunned for 1 round) will open up giants to sneak attacks. These are AoE spells so if you can hit multiple giants one of them is bound to fail.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.