Collectivism and the Inertia of Static Defense
The end of the Second Dark Age saw a massive shift in the way wars were fought. Rather than armies, small groups of soldiers in power armor and later mecha would duke out their battles, often to little consequence compared to the defensive capabilities of arcologies, with their ability to affix weaponry and function as castles in the 24th century. The mode of warfare had shifted far enough towards armor and defense that offensive weaponry had to make a transcendent jump in technology. This happened in two different directions, the flying warship and the assault mech.
The Assault mech is a staple of Battletech and in the fluff, the giant machines are able to turn the tides of entire campaigns and their appearance on a battlefield can send an enemy running for their lives. Except that outside of the fluff, the assault mechs are not that much more heavily armed that heavy mecha, and often ponderously slow, and have a myriad of other problems that are in place to balance out the game. That being said, the Cosmic Era isn't about balanced gameplay. The assault mecha in the Cosmic era live up to the fluff text, but they are 'balanced out' by the inclusion of levels of engagement. Assault mecha are really big and really badass, but compared to the aerial cruisers and warships they are not a major threat.
The assault mech is a vehicle on an entirely different scale of construction than the traditional battlemech. The smallest assault mechs tip the scales at a hefty 300 tons, and go up to nearly 1000 tons each. These behemoths mount the same average firepower of a Cosmic Era destroyer, or a contemporary cruiser.
The AWS-8 'Awesome' is a standard issue assault mech deployed by the Atlantic Federation. The mech is a walking particle cannon battery famed and feared for it's ability to 'Light up the Sky' with it's three medium naval grade particle cannons. These mechs are not very fast, and most commonly are used as mobile fortresses to anchor defensive lines and provide anti-warship firepower to ground based forces. They have been used to destroy fixed emplacements, and as ambush units to shoot down enemy destroyers and patrol craft.
The GOL-1 'Goliath' is a regular issue assault mech used and exported by the Eurasian Alliance. It is a 500 ton quad legged walking fortress armed with a medium naval grade particle cannon, missile launchers, and an internal bay to accommodate a platoon of power armor. The EA uses Goliaths to smash through defensive lines, deploy its armored troopers and support them with heavy gunfire. The Indian Army uses Goliaths to deploy conventional infantry, hoverbike units, and even carry smaller mecha into battle.
The STK-3 'Stalker' is the main model assault mech used by the ACPS and is primitive in its walker/pod assembly but makes up for this by the sheer amount of weapons it carries. The mech is able to mimic the Awesome's ability to light up the sky with optical lasers as well as being able to create a wall of missiles to destroy inbound aircraft and airborne light mecha. The heavily armored beast was designed for sustained advances, and moving through contested terrain, be it forest, jungle, or urban. In ACPS one of their core doctrines is the Dragon's Fist, a formation of 3 to 7 Stalkers advancing supported by following infantry and power armor troopers.
The AS-7 Atlas is the pride and terror of the Atlantic Federation, the massive machine carries a magnetic accelerator cannon that has been designed to bust foundations to topple arcologies, shatter bunkers, and reduce airfields, mechadromes, and other major structures to rubble, and it constitutes the largest weapon system placed on a individually piloted machine. Supported by lasers, machine guns, and a short range cruise missile launcher, the Atlas is not designed to engage other mecha, but military formations and cities. By itself. So far very few of these massive mechs have been produced and most are kept in ceremonial and morale tours.
Production and Deployment
The battlemech is the product of the Cosmic Era, its compromise between the material wastefulness of creating tanks and armored vehicles in the tens of thousands. They are vertical weapons in a vertical world. Where the average mech is 35-70 tons, the assault mecha are 300-1000 tons, they are the sort of wastefulness that was endemic to the Petroleum Era, so they are accordingly rare in number.
Most Assault mechs are produced not assembly line style, but in place, limiting their speed of production to a handful a year and a small number of facilities that are capable of producing machines so large. This limits their value in the battlefield as the monsters represent a much larger economic investment than even an equivalent tonnage of conventional mecha. Few commanders are willing to risk the massive machines, as in addition to the potential cost if the machine is destroyed, the impact on morale should one of the behemoths fall cannot be overestimated. The largest economy on the planet, the Atlantic Federation has in the vicinity of 120-150 assault mechs at any given time, and these billion dollar beasts are accorded personalities, and names.
The Panama Canal War proved that a victory can swing to a defeat over the disposition of the assault mecha. The Canal War was less than two months old when Latin forces made an overland surge during especially bad weather. Under hurricane force winds, the Federation sky fleet was withdrawn, or forced to elevations above the storm bands, leaving the ground forces to their own accord. The Latin army advanced with their forces spearheaded by a trio of AHN-1 'Annihilator' assault mechs. These formidable machines, armed with four 200mm short barrel assault cannons decimated the initial Federation ground forces and almost broke through the Panama City defenses. The city was damaged, but the mecha were not well supported as they advanced. The Federation garrison commander had targeted the Annihilator's supporting mecha, destroying those machines and allowing Federation assets to flank the trio of assault mechs. A squad of mixed Federation mecha caused extensive damage to two of the assault mechs, but sustained heavy loses in the process, and the garrison commander was moments from calling a fighting withdrawal when one of the Annihilators was toppled. This shocked the Latino forces and caused their ground forces to hesitate long enough for the Federation forces to fall back, regroup, and launch a counterattack.
Though all three annihilators survived and withdrew, the moment shift from one machine going down cost them the battle, and once the hurricane cleared, the Federation air and space assets eventually turned the entire campaign in their favor.
Logistics and Nightmares
Supporting Assault mecha in combat is a nightmare for ground crews as maintaining the beasts is expensive and time consuming. Given their small numbers, there aren't regular parts depots to keep them armed and repaired, and othen the weapons systems used by the assault mecha are unique to the mech, so ammo is a speciality ammo, such as the Atlas' 380mm MAC. The size of the mecha also produces a problem, as the parts themselves are simply monstrous. The legs of the aforementioned Atlas are each the size of a Federation Warhammer heavy mech. Logistics costs are typically 8 to 10 times the cost of conventional mecha, and repair and servicing times are likewise 5 to 20 times longer.
This high cost has prevented major proliferation of assault mecha, as smaller governments are typically unable to mount the resources to fund an assault mech program, built and maintain the machines. Some nations have purchased export models, but rely on outside contractors and suppliers to keep the machines running. India relies on the Eurasian Alliance for it's maintenance of its assault mecha, and the Kingdoms of Scandinavia rely on the Federation for their support. Smaller factions like Armas, Amerikka Command, and small unaffiliated nations are barely able to mount conventional mecha, let alone the true titans.
Prestige and Loyalty
Given the firepower and capabilities of Assault Mecha, their pilots are chosen with the same care of capital warship captains. A single assault mech in the hands of a traitor or terrorist could kill tens of thousands in the time it would take to mobilize a strike force capable of neutralizing the machine. As such, being assigned to an assault mech is the highlight of a pilot's career, and only the very best of the very best ever make to the biggest chair in the ground arsenal. These few are only chosen from mech pilots who have years of exemplary service, and are noted for their loyalty, patriotism, and numerous background checks.
The Assault Mech is the largest weapon system deployed on the ground, and the last word in the middle tier of engagement, able to destroy other mecha with ease, able to credibly threaten megastructures, and even in groups prove a deterrent to aerial warships. That being said, most assault mecha mount an equivalent amount of firepower as an aerial destroyer, and are only the very bottom level of capital scale level of engagement. While the Awesome can light up the sky with its particle cannons, a warship unloading it's salvos of full power naval grade particle cannons is going to be visible from space and detectable several hundred miles around.