As simple as it sounds, you need to win 103 matches. You win matches by destroying the enemies castles and then hitting their Warlord with a fireball. All of the matches that you win count towards this, except for the tutorial match and you can set up local 2-player games in the quick match game type and have the second controller idle while you win the match with your main account. Doing this will still take some time, but it is likely the quickest way of getting this achievement done.
Go to multiplayer from the main menu, then local match and choose these options;
Mode: 1 on 1
Stage: Random (This is not important)
Then on the next page sign in a second account and have them join as player 2. When the match begins, move the second players shield to one side of the castle and grab the fireball with your main controller. Aim directly at the center of the second players castle with a charged shot and the fireball will bounce back directly at your shield. Grab the fireball again and repeat this until you breach the second players wall, at which point send the fireball into the gap, winning the match.
This takes roughly 30-45 seconds to get a win and is undoubtedly the quickest way of boosting this achievement. Upon winning your 103rd match the achievement will unlock.