The Fatal Circuit is a boss that can be summoned by placing the Inactive Robot and interacting with it.
Unlike the Penguin UFO boss, the Fatal Circuit behaves similarly to a normal monster, simply chasing the player and trying to attack him/her, and does not seem to break blocks. As a result, strategies that can be used against the Fatal Circuit are simpler and more numerous than those for the UFO, in spite of the Fatal Circuit's much larger amount of health:
1. Requires a weapon capable of hitting through walls. This method involves building a wall 25 blocks high by 1 block wide next to the inactive robot (who can jump extremely high) in a relatively solid material such as blaststone. Place blocks to form a lip on top of the wall to prevent the Fatal Circuit jumping over. Finally, drill a couple of holes through it, to maintain line-of-sight to the Fatal Circuit. Once the Fatal Circuit is activated, it will try to attack the player, but will not be able to because of the wall: the player can then kill him at his or her leisure.
2. Another strategy involves building an arena-like area to fight the boss in.
Have walls spaced about 45~ blocks apart, about 30~ blocks high, and use platforms, preferabbly of dirt or some other cheap material, to span between the walls. Having multiple layers of these platforms, so that you can change your elevation at will against the Fatal Circuit, is beneficial.
3. You can also simply build a long runway type platform, which you can run backwards while firing your guns, keeping the Fatal Circuit at bay. This may require you building the platform higher off the ground, or simple digging through any obstacles in your way. While not very space efficient, it's a simple technique. Note that bows are not advised for this strategy since they slow the player down considerably while charging them, and the Fatal Circuit - for obvious reasons - does not drop meat.
3.a. Since the Fatal Circuit is not exceptionally fast, the above strategy can also be used with a sword or other melee weapon as long as it deals substantial knockback. However, melee weapons may leave the user open for a hit from the Fatal Circuit's flamethrower if he/she does not promptly knock the Fatal Circuit back as soon as it comes in range.
4. Dig a 2x2 hole in the ground, crouch in it and with an over-head melee weapon attack it when it comes close, you should be able to kill it without taking any damage.
5. Make a tall hole in the ground and put wooden platforms on it, put the Fatal Circuit on the wooden platform , summon it and destroy the platforms and then you should be able to easly kill it with a bow or a gun
6. Summon the Fatal Circuit on a pirate ship(easier to put it inside than on the deck).
Charge - Same as a monster's charge, the Fatal Circuit simply tries to ram into the player, dealing damage.
Flamethrower - The Fatal Circuit shoots flames a moderate distance towards you, dealing damage and inflicting the burning status.
Jump - Not an attack, but still something to be aware of - in spite of its large size, the Fatal Circuit jumps abnormally high. Keep this in mind when building traps or the like for the Fatal Circuit, and if trying to fight it hand-to-hand.
Note: None of its attacks seem to be able to damage blocks, meaning that the Fatal Circuit is very vulnerable to traps.