Mind you, a beetle, for example, would take forever to kill, but it's very doable and, since the PCs should all be able to fly faster than the beetle can move, and to damage it from outside its reach, it's just not a credible threat. Actually, looking at the poor beetle, it's painfully easy to kill if you're not pressed for time: for all its amazingly high armor class, unexplained spell resistance and over the top AC, it
has no energy resistance. Anyone with a
fly spell, a bag of holding full of alchemists fire, and a whole lot of patience can kill it. (For reference, it would take an average of 784 alechemists fires to do the job, which would cost 15,680 gp

, so there's probably a cheaper/more efficient way to do it).