Slow and expensive, but boasting enormous offensive and defensive capabilities, Mega Tanks are the ultimate in ground-based firepower.
The Mega Tank is a unit that deals in extremes. Its Mega Cannon is capable of completely obliterating any land unit in one shot even on Plains, save for other Mega Tanks. They will need to be close to full HP to do this to Md. Tanks and Neotanks. Against lighter-armored vehicles, Mega Tanks can still destroy whole units when they are at 6 or 7 HP, depending on terrain. In battles where naval units are relevant, the Mega Tank is also the only direct ground unit capable of dealing serious damage to most ships, although this will rarely be important in the vast majority of games. Their Machine Guns are very effective against foot soldiers as well, although using Mega Tanks to combat Infantry is extreme overkill and generally a waste of the Mega Tank's turn. Their Machine Guns are also no more effective against Copter units than a Neotank's, so using them to attack copters is inefficient.
Similarly to its raw offensive power, a Mega Tank has sky-high defense, taking low to middling damage from any other unit, again save for other Mega Tanks. Even a Bomber, considered the strongest anti-ground tech unit available, will still only deal around 40% damage to a Mega Tank even with a Comm Tower boost. Against targets that it can counterattack against, this means that even getting the first strike can lead to taking much more damage on the counterattack than is dealt to the Mega Tank.
This firepower and defensive potential comes at a cost, however, both in terms of funding and flexibility. The Mega Tank is by far the most expensive ground unit in the game, tied with the Battleship for the second-most expensive unit overall; on most maps, saving up this much money is simply not possible without introducing deficiencies in the rest of your army, reserving its use mainly for larger maps with more properties or very long games where a low-impact turn can be more easily absorbed. With an ammo capacity of only 3 shots before needing to resupply, a Mega Tank can easily run out of ammo if not careful, generally requiring nearby Cities or APCs to stay combat-ready. The Mega Tank is also a very slow unit, being the slowest vehicle in the game along with the Missile and taking a considerable amount of time to reach the front lines. In Fog of War, the Mega Tank is effectively blind like other high-cost ground units as well, being incapable of supporting itself and requiring other units to provide vision.
Due to its high firepower, defenses, and heavy dependence on supporting units, Mega Tanks are often best built as a breakthrough unit or for defending a front. Their immense damage output can allow it to punch holes in a defensive wall to allow more nimble units access to the backline, or their immense defense can let it function as a wall of its own, especially on a property, able to absorb lots of punishment and capably defend against an overwhelming number of enemy land units by itself.
Mega Tanks are extremely powerful units, taking quite a few units firing upon it in unison just to be able to take it down. Attacks from direct units while it is at full health are generally inadvisable due to the heavy damage they will take on the counterattack - indirect units do not do much damage either, but it cannot retaliate against them, so they can prove useful in whittling it down to a more manageable state.
Air units are generally good for fighting Mega Tanks, as they are still defenseless against Bombers and Stealths. B-Copters can be cost efficient at fighting Mega Tanks as well, being able to take a few hits from them while being significantly cheaper.
Overall, the best tactic against Mega Tanks is to use its very slow speed against it. Being slow units, Mega Tanks are very poor at picking optimal engagements, instead just opting to throw its high stats around to compensate for this particular weakness. Forcing engagements in areas that the Mega Tank cannot reach quickly will allow you to take out its supporting units before the Mega Tank itself can pose a threat. Without units to support it, a Mega Tank is a fairly easy, if very sturdy, unit to eventually overwhelm.