This achievement is easy enough to get in any game mode. If you still don't have this after playing through campaign, try a custom command room game on random with you and an AI player on any map. When the objectives pop up, check if there is one for capturing an enemy capital. If there isn't, exit and set up another similar game. When there is one, on your first turn, place all your troops into the territory closest to the enemy capital and take it over. Voila, quick and easy.