Probably one of the easier ones to do this on is the 8th Mission, Lynchburg. Progress through the level normally, and get all the way to the last artillery position. Get in the warehouse right before the cannons, and look outside towards the cannons. Walk outside and take a left. Go left to the fence, and turn left again. Up here is a glowing red barrel which you must walk over. Do this, and you will complete the bonus objective for Planting explosives at the warehouse. Achievement will unlock after the mission.