The steamrolling tactic consists in en masse building of a few complementary armored types, and sending them rampaging straight away onto the enemy. Its sole advantage is its sheer brute firepower. It's also plain fun to smoke the enemy in an overwhelming mass of units. The downsides are, however, numerous and annoying. It is incredibly time and resource consuming, provides for completely inflexible formations, and makes easy prey for any area damage weapons. As a matter of fact, this tactic is only an option in a team game, and then only when you have already eliminated an opponent and taken over his resources.
Allies
Mixed assault - A mix of Cryocopters, Apollos, Peacekeepers, Javelin Soldiers and at least 12 Guardian tanks will offer you decent results.
Amphibious assault - On sea, swap your Ground units for at least 5 Aircraft Carriers and 12 Assault Destroyers. Mirage Tanks and Athena Cannons are another powerful option on steamrolling your enemies as well as Century Bombers.
Three Pronged Assault: Air, Land and Sea - For this technique, you should use the Time Bomb and Surgical Strike protocols.
A good option is to load up four Century Bombers with 10 Peacekeepers and 10 Javelins. Build two Aircraft Carriers, at least six Assault Destroyers and two Hydrofoils. Pull your carriers and their screen to nearby the enemy base. Allow their squadrons to auto-attack defenses and structures. When they have cleared a path, send a scout so you can use your protocols to further weaken the enemy. When you feel confident your bombers won't get shot down, have them attack the enemy Construction Yard. Then paradrop your forces into the base, while simultaneously sending in your Assault Destroyers to provide support to the ground troops. You should quickly decimate the enemy base using this method. If you're on a land-locked map, use Athena Cannons instead of Carriers, and Guardian tanks instead of Assault Destroyers.
Soviets
The most suited faction for steamrolling, as their "mass production" upgrade reduces production costs by 25%. Also to their advantage is their ability to build simultaneously multiple tier 2+ factories without the need of individual upgrades.
Mixed assault - Swarms of Twinblades, MiGs, Bullfrogs, tanks and artillery units will serve you well.
Apocalypse Tanks - Building huge battalions of Apocalypse Tanks is another option for steamrolling, if money is no problem for you.
Hammer Tanks - A cheaper alternative to Apocalypses. Build 12–15 Hammer tanks and go to the enemy's base. Crush the infantry and Leech Beam down enemy vehicles. You get their weapons to eliminate aircraft. Leech Beam also is quite good against structures – your Hammers get a health boost and the structures go down at the same time. Although is if your enemy has swarms of anti-tank infantry/aircraft, your Hammers won't make it.
Empire of the Rising Sun
Naval battles are often won by few Shogun Battleships with Tsunami Tank and Sea/Sky-wing support. Remember to use the Ramming Speed ability to quickly destroy enemy ships.
Ground assault - Building Tsunami tanks, Imperial Warriors and Tankbusters (10-12 each of them) backed up with Jet Tengu support will serve you well, along with King Onis and Wave-Force Artillery.
Air assault works with a mix of Chopper-VXs, Jet Tengus, Sky-Wings and Rocket Angels (with their special ability on).
Tsunami Tanks - A huge battalion of Tsunami tanks also works. When anti-tank infantry is spotted, activate Nanodeflectors and run over them. Beware of enemy aircraft. Since Tsunami tanks are amphibious, they serve well at naval battles.
Rocket Angels - Rocket Angels' special ability makes them excellent steamrollers while they are already effective against vehicles and aircraft. Advanced rocket launchers upgrade obviously makes them even more efficient since their firing rate is improved.