Re: Clerics
"Command" is a great idea: many commands (e.g., "Die!") deny an opponent their dex bonus, allowing rogues to sneak-attack. A command to "Die!" can give the fighters the opportunity they need to approach the giant without incurring an AoO. Mass-command, of course, is great against lots of giants.
Rogues will probably want to use missile weapons: not only do their low hit points make melee with a giant very dangerous, but they also may have trouble reaching a giant's vitals when in hand-to-hand combat. Improved invisibility is almost a must-have for a rogue fighting giants.
Consider having grenadelike weapons available, to help overcome a giant's tremendous natural armor bonus.
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. A rusting grasp spell may also prove useful, since giants generally wield weapons too large for the party to use anyway. But the touch range of rusting grasp makes it risky. They should keep their animal companions out of combat, unless the companions are under the influence of Animal Growth. Druids have access to fire and cold spells, and should prepare them if fighting the appropriate kind of giant.
Finally, druids have some great movement-limiting spells, e.g., spike growth. If you can get most of the party into the air and prepare the battlefield with these spells, you can make it much harder for the giants to bring their melee power into play.
Daniel