So if I am understanding the situation right, I need to get to a floor above me to reach the minister, right?
If so, would it be ok to not go through the elevators at all? I know I cannot transfigure parts of things, but I figure Billy would know a "cut a hole in a wall" spell, and could cast it on the ceiling, then push himself upwards/transfigure himself into a bird and fly up, repeating until he's on the right floor to reach the minister.
If that doesn't work, how do you feel about people turning things into antimatter? Are there ready-made transfiguration spells that could turn, say, a dust mote weighing fractions of fractions of a milligram into antimatter? One milligram of antimatter would produce ~4.3 tons of TNT, so to have actual combat application it would need to be thousandths of that mass, which would get tricky, not to mention the world-changing applications of being able to transmute chirality at will [but that seems like something that would not be shared with muggles even under the loosest secrecy laws, for fear of antimatter warfare]
If I can neither blow them up with transfigured explosives nor just avoid them and go through the ceiling, I would like to bore through this corner and attack them from behind their barricade. It would probably lead to some casualties, but at least it is better than nothing. If that's not allowed, I think the easiest answer would be a barrage of Avada Kedavra, depending on if I could predict a reliable and safe exit vector for them, but that would require a detailed map and measuring spells at the very least, seeing as I would need to adjust for the depth of the ministry, the angle of the cast spell and the distance to the north sea, if it is even possible to avoid other underground populated areas, like deep basements or what have you.