Marooners

This is a cumulative achievement that requires you to play in Party mode, attack an opponent that currently has coins (points), then picked up the dropped coins. Attacks can be made with your melee weapon or the boxing glove, but a bomb just outright kills folks so they aren't useful here.

My recommendation is to play with a second controller or online with a friend, and hope for the stage "Twin Temple". This mode is hot-potato; the statue torch gives increasing amounts of coins, but then blows up. Have your opponent face a wall, then just wail on their back. This will prevent the torch from being grabbed by you, as well as providing many points that you can then steal. Make sure not to be too close after the flame goes out, in case you can get hit by the explosion too. Then make sure to grab as many coins as you can before the stage ends.

If you're unlucky and don't get Twin Temple, any stage that is small in area, has walls (so as not to fall offstage), and has respawning gems can work well. Have them grab some points, then attack them and try to get some of the dropped coins.

To see your current progress, go to the "Statistics" option on the main menu. Under "General", your current coins stolen is listed under "Points Stolen" (third from top). Once you get 100 points stolen across all Party games played, then the achievement will unlock.