This achievement requires you to win 30 multiplayer matches. It does not matter whether it is a player match or ranked match.
To boost it, create a player match with only slots for only 2 players and 1 private slot. Pick the first map. When the other player joins and you move to the next screen, change countdowns and item limitation to "no" (item limitation is optional; you will only need to select "no" if the non-winning player wants to fool around while they are waiting for the other player to win). Give each of yourselves 1000 G to start.
The first winner buys a quick knife in the first village, barrel armor in the second, and spike shield in the third. The other player does nothing (or something if they are bored) The winning player then levels up to level 7 or 8. They will then fight the Evil Lord and become the true hero. Both players vote for a rematch. Then they switch roles, other player wins, switch again and so on and so forth.
Assuming neither player has any multiplayer wins and both are going for the achievement, the achievement should take anywhere from 1 to 2 hours to boost.
Please leave comments on how I can improve this solution. I am always open to suggestions.