So just for the heckuvit, I've tried statting this up as an Elder Eidolon Elemental Mage (Ken-Li), but it looks a bit weak for CR 13?

Size/Type: Large Construct
Hit Dice: 12d10+30 (96hp)
Initiative: +5
Speed: 50 ft. (10 squares)
Armor Class: 30 (-1 size, +5 Dex, +12 natural, +4 deflection), touch 18, flat-footed 25
Base Attack/Grapple: +9 / +22
Attack: Gore +17 (1d8+9) or slam +17 (1d4+9) or large greatsword +18 (3d6+13)
Full Attack: Gore +17 (1d8+9) or 2 slams +17 each (1d4+9) or large greatsword +18/+13 (3d6+13)
Space/Reach: 10 ft./10 ft.
Special Attacks: Insanity aura, wreathed in flame
Special Qualities: Construct Traits, damage reduction 7/adamantine, darkvision 60ft., fast healing 10, immunity to magic, low-light vision, shared strength
Saves: Fort +4, Ref +9, Will +4
Abilities: Str 28, Dex 21, Con --, Int --, Wis 11, Cha 1
Skills: -
Feats: Cleave, Great Cleave, Power Attack and Weapon Focus (greatsword)
Environment: Any
Organization: None
Challenge Rating: 13
Treasure: None
Alignment: Always neutral
Advancement: -
Level Adjustment: -