Use high attackers such as Yuki or Tomomo if you have unlocked her, stack your deck with all the most powerful attack boosting cards you have and play on a map with the Battlefield field effect on. If you have DLC 7, Star Breaker will get very close to this achievement when attempting to win, as she can only pick win normas, meaning by the time she can win, she will have 14 wins, leaving 7 more. It is still recommended however to play on a map with avoidable bases to prevent accidental winning before you hit 21, for instance Tomomo's Abyss. You needn't win the match, just reach 21 wins and the achievement will be awarded.
The requirements for this achievement were changed in Version 1.8.6. The change allowed for earning 21 Wins instead of 30 Wins, decreasing the requirements for players who had not previously completed them.