An assortment of alien adolescents accompanying adult aliens on an alcubierre-drive ship just happen to have been passing near the Sol system. Having noticed that there is lots of EM radiation coming from the third planet, their ship was stopped so that its scientists could gather data. Firing up their quantum computers, some of the digital transmissions were decrypted. The aforementioned alien adolescents saw one of the decrypted transmissions, which happen to be a broadcast compilation of alien/monster and modern warfare movies, in particular, but not limited to, movies like Godzilla, Super-8 and Cloverfield.
Having entirely missed the point (since they neither read nor speak English) that the movies were intended to be entertainment, the alien adolescents have misinterpreted them as documentaries recording what they see as humanity's barbarity, celebrating warfare, the destruction of hapless lifeforms and the unjust capture and escape of an alien pilot and its ship.
The alien adolescents have resolved to show those arrogant humans what a real giant monster attack is, and to that end, they have designed a giant robot, a snakebot with a circular cross-section 1 Zode-la in diameter (1.19 km), and 2xPi Zode-la in length (7.48km), plus a 1.5 Zode-la (1.79 km) tapering tail (9.27 km long in total) that can mate with the snakebot's similarly-shaped mouth, forming a 4 Zode-la (4.76 km) high torus that can move in a rolling fashion, as well as being able to move in other serpentine methods when not hooped and rolling.
The robot has a maraging steel core and mechanism, with six equally-spaced hydrogen-fusion power-plants along its length for redundancy, any one of which could power the entire bot. The outer 1 Zode-gu (1/8 Zode-la, or 224 metres) is armour, made from overlapping, flexible arrangements of depleted Uranium-Tungsten alloy plates coated with Boron-Carbide.
To build all this, the alien adolescents simply program a 1 Rel-ek (3.84 liter) capacity atmospheric-descent-container full of versatile programmable construction nanites, which will self-replicate until there are enough to build, operate and maintain the snakebot.
Fortunately for the humans, these aren't military nanites, which are chock-full with weapons designs, strategies and rod-logic brains, just one of which could be expected to render Earth ripe for conquest (for whatever value of "conquest" is chosen) all by its lonesome. No, these are just engineering nanites, individually only about as smart as a human, and only with fairly basic programming. Unless given about the same time as a human committee, these nanites won't be coming up with too many innovations.
Hearing their seniors approaching near the end of their design conference, they jettison the incriminating nanite pod. The container lands on Antarctica, and the nanites immediately start building the snakebot. The better part of a decade later, we have 8.3 or so cubic kilometres of snakebot, weighing at least sixty billion metric tons advancing on the cities of the world, with the objective to destroy all human infrastructure.
The alien adolescents' goal is for their snakebot to flatten all human infrastructure more substantial than a tent, and to generally knock humans back to something approximating the stone-age.
Since the alien adolescents intend to be slightly more sporting than the humans in the "documentaries" seemed to be, the engineering nanites are programmed to build only one snake-bot, and to self-destruct, individually if separated or captured, or collectively, along with the snake-bot if possible if the bot is mission-killed or achieves its objective. On the other hand, they want to protect their snake bot until it completes its mission.
Given the likely capabilities of a species capable of building an alcubierre warp-ship and rod-logic nanites, what offensive and defensive weapons might the alien adolescents (who can be considered to be about as intelligent and well-educated as the users of WB SE) build into the outer armour of their snake-bot designs, considering their goal and that they have seen fairly recent movies containing images of modern human weapons - including atomic weapons - in use, and can speculate on their probable design and function. What use would each of these weapon systems be expected to fulfil?
Since the alien adolescents know nothing about human biochemistry, toxins are off the table, save as an accidental side-effect of a physical weapon system.
Any weapon deployed on board the snakebot must address the matter of resupply in the field, though remember that there will be quite a few nanites hanging around in the bot to act as its brains and self-repair mechanism, that could be repurposed to making ammunition if necessary.
Since this is [science-based], we aren't having any hand-wavium weapons here. If you couldn't say, or find out, how it works if asked, you don't get to include it in your answer, since the alien adolescents' schooling won't have covered it yet. That includes any futuristic technology required to make an alcubierre ship work.
In response to a comment by Gianluca, I recalculated the mass of this thing. My original calculation was that it weighed around 600 million tons. This has proved to be wrong, it actually weighs in at around 60.2 billion tons, assuming that it has 50% internal voids and that 5% of its armour is Boron-Carbide plating on top of Tungsten-Uranium plates/scales.