I do not wish to sound snide, but it's a waste to use engineers against such a frail target as a Marder. That tin can can barely withstand a hand grenade, and most of the models don't even have an MG to ward off pesky meat puppets.
If your troops enjoy a superior cover contrast (your troops in a building, the tank on an open street), you might want to just set them a cover arc so that they will attempt to target the tank just when it is passing by.
If you set the tank as a target manually, be sure to give your troops a run/advance order away from the tank, deeper in to the building. With a proper delay set with a pause command, your troops will attempt to chuck explosives at the tank once or twice, and fail or not, they'll withdraw to safety in the end. Giving a follow order will likely lead to catastrophy, especially if the tank is nimble and wary of contact.