Updated.
Here's a start for Alcyoneus...
Alcyoneus
Colossal Giant
Hit Dice: 45d8+585 (787 hp)
Initiative: +0
Speed: 40 ft. (8 squares)
Armor Class: 36 (-8 size, +27 natural, +3 breastplate), touch 2, flat-footed 36
Base Attack/Grapple: +33/+72
Attack: Oversized
+3 longspear +51 melee (8d6+37/x3) or slam +48 melee (4d6+23) or rock +26 ranged (6d6+23)
Full Attack:
+3 longspear +51/+46/+41/+36 melee (8d6+37/x3) or 2 slams +48 melee (4d6+23) or rock +26 ranged (6d6+23)
Space/Reach: 30 ft./30 ft. (35 ft. w/longspear)
Special Attacks: Rock throwing
Special Qualities: Low-light vision, resistance to acid 5, rock catching, specialized powers (born with armaments, oversized weapon, 2 more)
Saves: Fort +37, Ref +15, Will +15
Abilities: Str 56, Dex 10, Con 36, Int 8, Wis 11, Cha 12
Skills: 48 (see below)
Feats: 16 (9 can be epic)(see below)
Environment: Any
Organization: Solitary
Challenge Rating: 25
Treasure: Standard (in addition to arms and armor above)
Alignment: Usually chaotic evil
Advancement: 46+ (Colossal)
Level Adjustment: —
Born with Armaments: Alcyoneus was born armed with a +3 longspear and a +3 breastplate.
Oversized Weapon (Ex): The gigantes can wields a weapon sized for a creature one size larger without penalty.
I think we need to note in oversized weapons that it applies only to manufactured weapons, and not the gigantes natural attacks. What about thrown rocks, though?