They let that thing sneak up on them? And why is it attacking them from the side on the ground when they discovered it above them on the ceiling? It drooled on them, so it's straight above.
Basilisks usually don't spit acid the strength of xenomorph blood. Though if it's actual stomach acid, it is pretty strong. But not that strong.
Yeah, even if it can't be hurt by magic, it can be trapped by magic. Also, the magic shield works well enough, so in theory you should be able to use a blade or spear made out of magic shield to kill it.
Pretty sure that's not the weakest one.