Bankruptcy is something that happens quiet infrequently in this game. To make this achievement easier, just randomly select one player that you will be buying all the properties on the board with (always go to auction, resign with everyone and make a low bid with that one player). Play for while until a good bit of the properties are owned by the one player (two or three times around the board should be good). Then trade all of the money from the three other players to the one player. Then just play naturally until the a poor player lands on an owned property. If things work out, they will not have the money to pay the rent for landing on the property, and will go bankrupt, unlocking this achievement. The bankrupt player may need to hit the "Resign" icon for the achievement to pop.