TaranTheWanderer
Legend
Out of curiosity, have been used in an actual game and if PCs have actually fought these creatures. Or is this all theorycrafting?In my old 5e Epic Monster Updates I stated my Primordials as Kaiju sized monsters and made them in "parts." This is similar to @the Jester's suggestion. However, I also made a version of godzilla that was just a mythic statblock with multiple mythic stages. You could combine the two to make something truly terrifying!
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Godzilla by Vsales
Godzilla
Gargantuan dragon (kaiju), unaligned
Armor Class 25 (Natural Armor)
Hit Points 717 (35d20 + 350)
Speed 400 ft., swim 1,000 ft
STR DEX CON INT WIS CHA 30 (+10) 11 (+0) 30 (+10) 7 (-2) 14 (+2) 30 (+10) 
Saving Throws Str +19, Con +19, Dex +9, Cha +19
Skills Athletics +28, Perception +11
Damage Resistances acid, fire, force, necrotic, poison; bludgeoning, piercing, and slashing from nonmagical Attacks
Damage Immunities lightning, psychic, radiant
Condition Immunities frightened, poisoned
Senses Darkvision 10,000 ft., Passive Perception 21
Languages understands Common but can't speak
Challenge 30 (155,000 XP) Proficiency Bonus +9
Adamantine Weapons. Godzilla's weapon attacks are considered adamantine and magical for overcoming resistances and immunities.
Brutal. Godzilla's melee attacks deal an extra 140 damage of the attacks type on a hit (included in the attack).
Colossal. Godzilla is 350 feet tall and its space is 230 feet by 230 feet. Any creature without the Colossal trait can enter its space and must do so to hit it with an attack with a reach or range of 20 feet or less. Additionally, Godzilla has advantage on checks and saving throws to be pushed, pulled, or grappled by creature's without the Colossal trait.
Damage Threshold.* Godzilla is immune to all damage unless the attack or effect inflicts more than 20 hit points of damage, in which case it takes damage as normal.
(*Note: this trait is not factored in the CR, consider removing this trait if you wish to have PCs challenge Godzilla)
Giant Monster. When Godzilla moves, its space and the ground within 50 feet of it is violently rent. The area becomes difficult terrain and any Huge or smaller creature in the area must make a DC 27 Dexterity check or be knocked prone, loose Concentration. Structures in this area take 150 bludgeoning damage. Additionally, a creature in an area devastated by Godzilla, refer to Giant Monster, must make a DC 27 Constitution saving throw for each minute it spends in the area or become poisoned.
Kaiju (Mythic Trait, 10 per Rest). If Godzilla is reduced to 0 hit points, its current hit point total becomes 717, all conditions and effects it is suffering end for it, it recharges all of its abilities, and it can use its Mythic actions. Award a party an additional 155,000 XP each time Godzilla uses its Kaiju mythic trait (1,705,000 XP maximum).
Immutable Form. Godzilla is immune to any spell or effect that would alter its form.
Radioactive. When an attack or effect of Godzilla's inflicts the poisoned condition, it ignores immunity to the poisoned condition. Creatures poisoned by Godzilla loose 1 hit point from their hit point total every 24 hrs. A Greater Restoration or similar powerful magic can remove the condition.
Siege Monster. Godzilla deals double damage to objects, structures, and creatures of Huge size and larger.
ACTIONS
Bite. Melee Weapon Attack: +19 to hit, reach 40 ft., one target that has the Colossal trait. Hit: 342 (35d10 + 150) piercing damage. Each creature within a 20-foot radius of the target must make a DC 27 Dexterity saving throw or take the same damage as the target.
Claw. Melee Weapon Attack: +19 to hit, reach 75 ft., one target that has the Colossal trait*. Hit: 272 (35d6 + 150) slashing damage and the target must make a DC 27 Strength saving throw or be pushed up to 200 feet, knocked prone, or grappled, Godzilla's choice. Each creature within a 20-foot radius of the target must make a DC 27 Dexterity saving throw or take the same damage as the target.
*Godzilla can also use this attack against creatures on its body.
Stomp. Melee Weapon Attack: +19 to hit, reach 75 ft., one Gargantuan or smaller target or prone or incapacitated target with the colossal trait. Hit: 307 (35d8 + 150) bludgeoning damage and fall prone. Until Godzilla uses its Stomp again or moves, the target is restrained. While restrained in this way, the creature (or another creature within 5 feet of it) can use its action to make a DC 27 Strength check. On a success, the creature relocates to an unoccupied space of its choice within 5 feet of Godzilla and is no longer restrained. Each creature within a 30-foot radius of the target must make a DC 27 Dexterity saving throw or take the same damage, knocked prone, and restrained as the target.
Tail Attack. Godzilla can make on of the following attacks.
Breath Weapons. Godzilla can use one of the following breath weapons. Godzilla must charge its breath weapons for two full rounds, taking no other actions before it can use its breath weapons. A creature reduced to 0 hit points by Godzilla's breath weapons is disintegrated and dies. Additionally, targets that fail their saving throw by 10 or more (DC 17) take maximum damage from these attacks.
- Tail Slam. Melee Weapon Attack: +19 to hit, reach 400 ft., one target with the Colossal trait. Hit: 307 (35d8 + 150) bludgeoning damage and the target must make a DC 27 Constitution saving throw or be stunned for 1 minute. The target can make a saving throw at the end of each of its turns, ending the condition on itself on a success.
- Tail Sweep. Godzilla sweeps its tail in a 400-foot arc. Each creature in the area must make a DC 27 Dexterity saving throw, taking 307 (35d8 + 150) bludgeoning damage and knocked prone on a failed save, or half as much damage on a successful one. Godzilla needs a full round to prepare this attack, taking no other actions before it use its Tail Sweep.
LEGENDARY ACTIONS
- Radioactive Beam. Godzilla exhales a super-heated beam in a 5,000-foot line that is 30 feet wide. The first creature in the line must make a DC 27 Dexterity saving throw, taking 315 (30d20) fire damage plus 315 (30d20) force damage plus 315 (30d20) necrotic damage and poisoned on a failed save, or half as much damage on a successful one. If a target is disintegrated by this attack, the beam continues to the next target in the line.
- Radioactive Breath. Godzilla exhales super-heated flames in a 3,000-foot cone. Each creature in area must make a DC 27 Dexterity saving throw, taking 157 (15d20) fire damage plus 157 (15d20) force damage plus 157 (15d20) necrotic damage and poisoned on a failed save and be incapacitated until the end of its next turn on a failed save, or half as much damage on a successful one.
Godzilla can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Godzilla can use any unspent legendary actions at the end of each round and it regains spent legendary actions at the start of its turn.
Unstoppable. Godzilla removes one condition or effect it is suffering. It can do so if it has at least 1 hit point, even while unconscious or incapacitated.
Claw (Costs 2 Actions). Godzilla makes a Claw attack.
Regenerate (Costs 3 Actions). Godzilla regains 100 hit points.
MYTHIC ACTIONS
If Godzilla's mythic trait is active, it can use the options below as legendary actions for 1 hour after using Kaiju.
Bite (Costs 2 Actions). Godzilla makes a Bite attack.
Pulse (Costs 3 Actions, Recharge 1/Rest). After Godzilla's fourth use of its Kaiju mythic trait, it can create a wave of kinetic energy in a 1,000-foot radius centered on it. Each creature in the wave must make a DC 27 Strength saving throw or pushed to the edge of the wave and knocked prone.
When I see these kinds of stat blocks, I often wonder if PCs are actually meant to fight these and if it's even possible in 5e to survive even a single attack against things like this.
