Valkyria Revolution

You will need a sapper character (Sara or Blum) in your party for this achievement. Land mines are small circular discs on the ground that will explode when stepped on. Standing near them with your sapper, you can press to dispose of them. Explosives are the groups of red barrels that are strewn everyone throughout the maps, though I am unsure if all of them count, or just the ones where you're asked to blow them up for the mission objective. There are dozens of mines in most story missions though, so just keep disarming those until it unlocks, then you can just avoid them to save time since you don't get anything for disarming them unless it's an optional objective (though the rewards for those are minimal).