My first thought is that you definitely should check out the Feral Ravager (IIRC) from Librum Equitis Volume I. It's a PrC for creatures with Reach (like, say, Giants) that specializes in controlling the battlefield by preventing the PCs from moving. The class has an ability to the effect of "if anyone in your threatened area tries to move (including a 5' step), you can make an AoO and hook them with your weapon (or somesuch) to prevent them from moving." That is usually a hugely nasty shock for PCs, especially when the Giant waits until their within his 10' reach, then prevents them from (a) closing to engage in melee because they only have a 5' reach, and (b) prevents them from fleeing.
I'm just going off memory there, I'm sure there are more goodies to it, but it's a NASTY ability against melee types when combined with Combat Reflexes.
So there's your Giant Leader's right-hand man - the guy who physically controls a battlefield by stopping the PCs in their tracks - literally.
The second thing to have handy is a sorcerer/wizard who can fire up an anti-magic shell. Physically, giants are going to be (more or less) superior to most PCs in a slugfest - it's magic that evens things up. Take that edge away, and have the giants prepared to fight with just their sheer mass, and you should be able to do some serious damage. If you don't like that idea, try a dispel-magic specialist to (a) dispel the PCs' buffs and (b) counterspell relentlessly.
Once you've got those two things in place - an anti-magic provider and a "hold the PCs still" type heavy (maybe with a spiked chain), all you need is a couple of well-armored, giant-sized greatsword wielding tanks with weapon specialization, improved critical, power attack, and so on to deal massive amounts of damage to the (hopefully) motionless, magicless PCs.
That is both nasty and to me, satisfyingly giant-ish... make the PCs and giants face off as the gods intended... the PCs put down their magic and the giants will put down their buffs they'll try to kill each other like civilized people.
Fezzik
"I can't help it always being the biggest and the strongest."
/Fezzik
--The Sigil
My 2 coppers.