Clockwork Anatomy

Legendary Resistance (1/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Composed of Parts. Each limb has health equivalent to 60. Once destroyed that limb can no longer function. The limbs do not share the vulnerability to Thunder damage. 
Left Arm: Can no longer use gassing weapons.
Right arm: Can no longer use the Slash or Gear Fist attacks, can no longer manipulate objects.
Legs: When one Limb is destroyed movement speed is halved, if both are destroyed movement speed is reduced to 0.

Multiattack.  The mech makes three attacks: always one with its aperture at the end of its turn and two with its hands.

Aperture Energy. Ranged Weapon Attack: The mech blast energy from its aperture in an 150-foot line 10 feet wide and 10 feet tall.+16 to hit, reach 10 ft., one target. Hit: 25 (6d4 + 10) fire damage.

Slash. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.

Gear Fist. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Gassing Weapons (Recharge 4–6). The mech uses one of the following gassing weapons.

Acid Gas. The mech expels acid gas in an 25-foot cone. Each creature in that cone must make a DC 19 Dexterity saving throw, taking 45 (13d6) acid damage on a failed save, or half as much damage on a successful one.

Sleeping Gas. The mech expels sleep gas in a 60-foot cone. Each creature in that area must succeed on a DC 16 Constitution saving throw or fall unconscious for 10 minutes. This effect ends for a creature if the creature takes damage or someone uses an action to wake it.

When the central torso is attacked a weaker blast of the Aperture Energy attack is released as a reaction, with a -10 damage modifier.

The mech can take 2 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. The dragon regains spent legendary actions at the start of its turn.

Detect. The dragon makes a Wisdom (Perception) check.

Aperture Release. The mech makes an aperture attack.