(OOC: I hadn't thought of that, but you're right--it doesn't say that the summoned mounts from Mount vanish when they lose all their HP)
*Tor conjures some mounts to block the Phantom Steeds from the sand. Shortly, after those mounts are slain, he has Gur skin them and use their remains to enhance the tent, thus managing to keep all of the Phantom Steeds protected, barely. It is a tight squeeze, and everyone is going to be aching in the morning from all this contortion, but they make it, and finally, the storm blows through and is over.*