Simple achievement
From the main menu, choose Fight - Arcade - Tag Match - Rookie - and then choose your 2 characters you would like to fight with. After choosing your stage you will start the match. The tag match is the same as a solo match but you obviously can tag your fighters in and out through the match. Being on the Rookie difficulty, I highly doubt you will have to tag your fighter out at any point. To earn this achievement, you simply have to beat both opposing fighters, and once you win, move on to the next round. There are only 5 rounds so 10 opponents total. Achievement will pop after the 5th round has ended.