In order to unlock this achievement you have to win 40 ranked matches (not 50 as the achievement tile implies) in total across 3 (or more) online modes.
Example: 35 wins in "Statue Defend", 4 wins in "Destruction" and 1 win in "Deathmatch".
You can either try to get 40 wins legit or you can boost. The only problem with boosting is that ranked online matches can't be limited to 2 players / 1vs1. Every lobby has room for 4 players. But as soon as 2 players are in the same lobby a 25 sec. countdown will start. If noone else joins in the meantime the game will start with only 2 players. So I'd suggest boosting in less popular online mode
s like "Statue Defend" or "Destruction" and leave the lobby if a stranger joins.
For the actual boosting keep in mind that there is no "Surrender" option like in previous Worms-Titles but I can confirm that if Player #1 quits it'll count as win for Player #2. Take turns in winning/quitting until each of you has won 40 times and the achievement will unlock.