A robotic being based on Emperor Demon Gava. It consists of a large, skull-like head and two long robotic arms with assembly claws on them. Its main method of attack consists in building small devices from the machines or scrap lying around with which it attacks intruders. Each device constructed behaves differently depending on its form.
After an exhilarating fight on the way to the research center, the heroes arrive just in time to save Dr. Geo, who informs them that the enemy is after the alien cell. After a short fight, they arrive to the room where the cell was stored, only to find out the containment device has been breached and the cell stolen. The room's exits suddenly shut down and an ominous mechanical figure slowly starts descending from the ceiling.
The Master Builder is a gigantic robot with two large, mechanical arms which he uses to smash the ground and to assemble an assortment of objects with which to attack the player. Each time he smashes the ground, big pieces of junk will fall from the ceiling and the monster will then proceed to form a random figure with them. These can only be destroyed after the figure has been finished (that is, when the pieces become red).
The boss will usually start the battle by building a big tower and then push it to either side of the room in an attempt to crush the player. In another attack, he will build two small figures which will then start "walking" around the room.
He can only be harmed by shooting him on the head. The far ends of the room provide safe spots for when the monster smashes the ground.
In Stage 7, the Master Builder reappears as a robot called Engineer-Demon Assembly Line Gyaba. It is a wall-mounted machine that consists of a robotic head (weak point) with equally robotic claw arms. The arms frequently block and protect the head, only moving away to build diverse attack devices. The object they build depends on how much damage the boss has received, being sentry bots, barrel inchworms, and jetpack turrets in that order. When a device, or group of devices, is destroyed, another one is rebuilt.
When the boss's health reaches below a certain amount, it stops building devices and aims its left claw arm at the player for a few seconds before launching it at them.
At this point, the boss attaches various arm weapons that attack briefly before being launched at the player. It attaches either a minigun that fires several bullet clusters, a rocket-launching cannon, and a large hammer arm that sends a shock-wave when it smashes on the ground. The boss cycles through these weapons in this order until it is defeated.
- The Master Builder's and Mother Alien's overall appearance and attack methods in conjunction resemble those of Emperor Demon Gava.