A combination attack can be performed when an enemy is in a dazed type of status. This includes all elemental attacks from the Majin as well as when you see the enemy with the black drops moving out of the body. An easy way to get these is to have the Majin attack an enemy and knock them to the ground The second time he knocks the enemy down, you need to run over and attack until the prompt shows up. You should get this by the end of the game. If not you will get it while grinding kills for experience at near the very end of the game (see "Brave One" achievement description for more details).
Note: Finish attacks will not count towards this achievement.