XM1 Excalibur Super-Heavy Siege Vehicle

After the humiliating defeat at Mannheim, especially at the hands of the seemingly invincible World Breaker, the UNSC faced a difficult prospect. If Mannheim was going to be recaptured, there would need to be a second showdown with the World Breaker. Its shield were almost impenetrable to conventional weaponry, and due to its position close to inhabited, anti-ship weapons and nuclear weapons were unacceptable. So, during almost immediately after the loss of Mannheim, specialists began development of a weapon to kill that beast. 
Developed hastily, and only ready in the final months before the Liberation of Mannheim, the XM1 Excalibur was intended to be the weapon to defeat that monster. Technically a weapons system consisting of several vehicles, their were two primary vehicles, codenamed Arthur and Merlin. Both were hastily refitted M510 Mammoth Siege vehicles, with Arthur, the lead vehicle, replacing its Magnetic Accelerator Cannon with a sub-capital grade ion cannon. This weapon fired a particle beam at near relativistic speed, which could cause massive electro-magnetic fallout, able to cause significant damage to energy shields. This weapon was mounted on a turret, though the particle accelerator needed to fire it took up the majority of the interior. 
The second vehicle, Merlin, carried the fusion reactor from the damaged Durham-class Corvette, UNSC Isle of Monsters, carried on the heavily modified vehicle to act as power source for the mighty ion cannon. It also carried a pair of anti-air cannons to ward off close ranged air assaults. 
The entire vehicle was prototyped, tested, and approved within two months, and deployed with the expeditionary forces sent to retake Mannheim. During Operation Giant Slayer, it got off a single shot after a fifteen minute charge, while the UNSC Army, Marine Corps, Navy, and Air Force conducted a massive diversionary assault. It managed to get one shot fired off, which knocked out it's shielding. With it's seemingly impenetrable shielding lost, the battle quickly swung in the UNSC's favour. It got a second shot off one the retreating vehicle, but due to the poor angling, it could only damage one of the weapon mounts. 
After the Liberation of Mannheim, the Excalibur was left to languish in a military stockpile, but during the Third Battle of Mannheim, the vehicle was quickly pushed into action as a defence against renewed combat. During this battle, it formed a lynchpin of UNSC air defence, shooting down two landing vessels during intense air assault. While it was eventually overrun, and forcibly abandoned, the vehicle had caused such horrendous damage to the attacking forces that the battle was ultimately a UNSC victory. The remains of Arthur formed part of the Mannheim War Memorial, constructed from vehicles destroyed during three previous battles to safeguard Mannheim.