My party faced off against a number of guards with AC16+ and a pair of rat swarm with an AC10. Guess which enemy was the problem.

Those rats were probably the best I've ever rolled as a player or DM. They made every save and I rolled max damage more than once.

I had the party tricked into a dead end where the humanoid fighters blocked one end and someone unleased the rats from the other. There was even an archer in a high window that took more than one turn to reach. Yet he eventually went down in two hits while the rats almost totally devoured the party's ranger who later lamented in a tavern that he's faced wild boars, gorillas, alligators, giant frogs, and axe beaks but it was the rats that came \*this\* close to doing him in. If he hadn't been rescued by the fighter's bait and switch maneuver it might have been over for him.

Then again, he was targeted by the rats because the other two logical targets ran away outside of range just about the time I was ready to go after them.