What is this monster's CR/weaknesses?

[RedWarlock]

So, I've got this major monster I'm throwing at my party (well not at, past, letting it go on a civilization-destroying rampage, and and letting them try to stop it, since they awakened it). The basic idea is that this WAS the regular Tarrasque, buried underneath Cyre, but the Mourning happened, with this thing at the center of the Glowing Chasm. The T itself condensed, absorbing energy from a massive dragonshard being experimented on by Cannith in that area, growing traits of a crystal dragon as well as other magical properties.

Straight up, it's a tarrasque, with the Half-Dragon (crystal), Spellwarped, and Dungeonbred templates added. (I mainly used dungeonbred to bring down its size, since I don't have any colossal minis, but I do have all the gargantuan dragons. Using Icingdeath as its mini.)

Listed CR after those templates is a 24. It's vaguely on the ball, though a couple factors have some high and low points. (the current party is about level 14, give or take. This is intended to be a campaign-ending threat that they will be able to fix plot-wise by the time they reach 20th level, give or take, by recovering a series of artifact macguffins.) Obviously it's a melee beast, though the breath weapon and wings give it a little more flexibility, and mean it can't just be spammed at range like a normal T.

Straight-up HP damage seems to be the main way to take it down, but the guys are very leery of getting anywhere close to even try. (the party is almost all casters, a shaper/constructor, a lich archivist, a necropolitan uttercold evoker, and a human rogue. The lich's player optimizes to the gills without even trying, while the rogue's player is an older player who tries but needs more help with optimization. The evoker and shaper optimize pretty well, but I curtail the more borked end of things.) They pretty much woke it up, tried one or two spells/powers on it, watched those fail on SR, took one overhead flying strafe with the breath weapon, then ran for the hills.

I'm debating a couple changes on it, though. One is using the base tarrasque's SR of 32, or at least something closer than the current 59, because even with Assay SR and other SR-affecting spells, they can't really reach that right now, if ever, short of going epic themselves. (Maybe 35-40?)

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The Beast That Sleeps                                CR 24
NE Gargantuan Aberration (Augmented Dragon, Magical Beast)
Init +12; Senses darkvision 60 ft., low-light vision, scent. Listen +50, Spot +50
Aura frightful presence 60 ft. (on attack/charge, will DC 37 Will save vs shaken)
AC 39, touch 10, flat-footed 35
   (-4 size, +4 Dex, +29 natural)
   mobility (+4 vs AoO)
hp 1080 (48d12+768), regeneration 40, DR 15/epic
Immune acid, fire, energy drain, ability damage, poison, disease, incurable/bleeding wounds
SR 59 (spell absorption), carapace (30% chance to reflect rays, lines, cones, and magic missile on caster)
Fort +38, Ref +29, Will +20
Speed 30 ft. (6 squares), fly 60ft (good); rush (150 ft. move, 1/minute)
Melee (Power Attack) bite +35 (4d8+51/18-20/×3) and
   2 horns +35 (1d10+40) and
   2 claws +35 (1d12+40) and
   tail slap +35 (3d8+40)
      or
Melee (normal) bite +65 (4d8+21/18-20/×3) and
   2 horns +65 (1d10+10) and
   2 claws +65 (1d12+10) and
   tail slap +65 (3d8+10)
Space 20 ft.; Reach 15 ft.
Base Atk +48; Grp +81
Atk Options improved grab (bite on H or smaller; free action grapple; -20 grp to not be grappling), Power Attack
Special Actions breath weapon (standard action; 45ft cone, 6d8 light, reflex DC 49 or blinded 1d4 rounds; 1d4+1 round delay), shock wave (full round action; 240ft area, bull rush +33 vs Str checks, knocked down; objects/structures take 1d6+21)
Abilities Str 53, Dex 18, Con 41, Int 9, Wis 14, Cha 16
Feats Cleave, Combat Reflexes, Dodge, Improved Initiative, Mobility, Power Attack, Flyby Attack, Improved Flyby Attack, Improved Multiattack, Superior Initiative, Enlarge Breath,  Improved Maneuverability, Improved Speed, Shock Wave, Spellcasting Harrier, Improved Toughness, Dragon Breath
Skills Listen +50*, Kn (Arcana) +9, Kn (Dung) +11, Kn (Nature) +12, Kn (Religion) +11, Search +39, Spot +50*, Survival +42 (46 following tracks)
Spell Absorption (Su) Whenever a spell fails to penetrate the Beast's spell resistance, the Beast gains one of the following benefits, chosen at the time that the spell resolves.

• Might: Gain a +4 enhancement bonus to Strength for 1 minute.   
• Agility: Gain a +4 enhancement bonus to Dexterity for 1 minute.
• Endurance: Gain a +4 enhancement bonus to Constitution for 1 minute.
• Life: Gain temporary hit points equal to 5 × the level of the failed spell.
• Speed: Base speed increases by a number of feet equal to 5 × the level of the failed spell.
• Resistance: Gain resistance 10 to one energy type (acid, cold, electricity, fire, or sonic).

Swallow Whole (Ex) Grabbed Huge or smaller; successful grapple check. Inside, 2d8+8 crushing damage plus 2d8+6 points of acid damage per round. Swallowed creature can cut out by dealing 50 points of damage to the Beast’s digestive tract (AC 25). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out. Gullet holds 2 Huge, 8 Large, 32 Medium, 128 Small, or 512 Tiny or smaller creatures.
Regeneration (Ex) No form of attack deals lethal damage. Regenerates even if fails a saving throw against a disintegrate spell or a death effect. If fails save vs spell or effect that would kill instantly, the spell or effect instead deals nonlethal damage equal to the creature’s full normal hit points +10 (or 1090 hp). Only slain by raising its nonlethal damage total to its full normal hit points +10 (or 1090 hit points) and using a wish or miracle spell to keep it dead. If limb or body part is lost, the lost portion regrows in 1d6 minutes (the detached piece dies and decays normally). The creature can reattach the severed member instantly by holding it to the stump.
Skills  *+8 racial bonus on Listen and Spot checks.