This can be done in Skirmish mode or Ranked Multiplayer (it cannot be achieved consistently in private or unranked multiplayer). To win an Annihilation game type, you must destroy all the enemy units: combat units, ACU, and engineers. You do not need to destroy their buildings. Annihilation is available on all 34 multiplayer/skirmish maps. This cannot be done by the opponent quiting out in a multiplayer game. The fastest method is to have a partner in multiplayer and set Fog of War to None, Game Speed to Fastest. Have one person build an air base and start pumping out Tech 1 bombers with the move set to right next to the enemy ACU. They will automatically start targeting the ACU when they get close, and the starting ACU can't target air units so it will just sit there and be chiseled down. This takes about 3-4 minutes per game.