I can't remember which is which, but there's an "On hit" effect, and a "Per attack" effect, and the Reaver only affects one of the two. Beaver (and Miss Jelly) both can't cause mass stuns/instakills from the Reaver.
Well give Beaver Mjoelnir and lots of pants for luck. He can chain stun 3 creeps at once. It all depends on proc chance i.e the stun should be exactly on 3rd attack. If timed you can stun 3 creeps. This really helps in crowd control. Got Till 3k seconds because my beaver supported carries by just stunning creeps like crazy.
Nice! If you want, Mjoelnir also works great or Mr. Iron if you load him up with tons of Luck/Speed, and then give him two Withered Items. He'll stop entire waves from advancing past himself.