For reference: Our giant currently has a -14 to Hide, without any bonuses. But then there's also this:
Camouflage (Ex): Since a GIANT looks like a normal hill when at rest, it takes a DC 20 Spot check to notice it before it attacks.
Having both a Hide bonus when immobile and a Camouflage Special Quality seems rather weird. When would you use one rather than the other? [Note that for Hide there needs to be *some* covere available. Camouflage strictly speaking doesn't require that, although it wouldn't make much sense to require a higher Spot check without cover than with...]
For simplicity, I would advocate doing away with the Camouflage, and folding it into a hefty Hide bonus when immobile. I'm voting for +8/+20. That would still put it at a Hide of only 14 when immobile - much more reasonable than the Camouflage ability.