With the entrance to your back, notice the two barrels on the right of the big gun.

You need to pull them away from there, so back up a bit and use the Grapple to pull them. As you pull the first one, you may explode both or none, so be careful. In any case, make them explode.

Notice the three boxes close to the entrance. These are movable. Push/ pull one of them to the place where the barrel were. Push the box against the railings.