Ambushing is the use of usually fast and manueverable units to attack an enemy with speed and surprise.
Allies
The Allied Mirage Tank is the ultimate anti-armor ambusher. It's combination of stealth and firepower is a highly dangerous foe. It can wait for an enemy to pass by, and then fry them with its spectrum cannons.
Soviets
The Soviets don't have a real ambusher on land, because they mostly rely on strength and attrition. However, the Sickle can use it's Flea Jump to jump on top of unsuspecting enemy infantry. The Mortar cycle from Uprising can also do some level of ambushing. At sea, the Akula Sub can suprise enemy vessels with it's deadly torpedoes.
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.