So, if this game gets ahead of me while I'm asleep, I just thought I should say that a tentative idea for using the river was freezing the surface of it enough to supports ourselves and luring the limo onto the ice in pursuit of us, where its weight would break the ice and cause it to sink. (Could then possibly freeze over the top of it again for good measure?). Of course, this depends on the level of control we have over our Freeze spell and the time involved and probably a few other things. And of course we have to reach the river, too.
Alternatively hoping we might find a pile of bricks or cinderblocks on the way so that we could try the Wall trick again with a harder barricade. Who knows what other logistical problems might present themselves in the meantime, of course.