Ambushing is the use of usually stealth, fast and manueverable units to attack an enemy with speed and surprise.
The Allied Nations
Mirage Tanks — The Allied Mirage Tank is the ultimate anti-armor ambusher. Its combination of stealth and firepower make it a dangerous unit. It can wait for an enemy unit to pass by, and then fry them with its spectrum cannons.
Spies - The spy can create havoc by bribing nearby enemy units to fight for you, and attack your enemy's units.
The Soviets have very few ambushing tactics, but mostly they rely on their brute strength.
Terror Drones were very effective at ambushing, due to the Terror Drone's speed. If a small group of Terror Drones could catch a armored column off guard, they could wreak havoc in seconds. The Sickle can use its Flea Jump to jump on top of unsuspecting enemy infantry. The Mortar Cycle from Uprising can also be used as an ambush unit, to some extent.
At sea, the stealthy Akula Submarine can surprise enemy vessels with its deadly torpedoes. The fully-amphibious Stingray are well-suited to surprise attacks and coastal raids due to it's speed and evasive maneuver, thus catching the enemy off guard.
The KA-65 Twinblade being a versatile aircraft can carry infantry or vehicles can drop them into thick of battle, thus surprised assault on enemy forces, this tactic is also good on a enemy base as well.
Empire of the Rising Sun
The Empire of the Rising Sun can use the transforming abilities of some of their units to their advantage. For example, a MiG might attack a Chopper-VX, and then the VX would switch to Striker VX form and blow the MiG out of the sky. Or a Cryocopter might start freezing a Mecha Tengu, then the Tengu would switch to Jet Tengu form and destroy the Cryocopter. However, this could be countered by a combination of units, such as Twinblades escorted by MiGs, or Vindicators/Cryocopters defended by Apollos, or Chopper VX's guarded by Jet Tengus.
Yari Minisubs — On Sea the Yari must surface to fire but in large groups can decimate the most strongest ships when they ambush the enemy ships.
Sea-Wings — Sea-Wings only need to surface to fire if they plan to ambush aircraft with their missiles or fly into the sky and shoot enemy infantry.
Tankbuster Ambush — Bury some tankbusters underground and when the enemy passes by, order the Tankbusters to pop out of the ground and ambush nearby vehicles.
Sleeper Ambush — Use this protocol at areas that are undefended and they will start up a ruckus.