Shatter is an encounter designed to be more than a bit of an homage to Splinter from TMNT. Shatter is a wererat who has been exposed to toxins, and constantly is seeing into the realm of madness. This has caused him to enraged, lashing out at those around him. Unfortunately for them, getting his saliva into your wounds will earn you a brief glimpse of that realm as well...
Shatter's encounter revolves around one main mechanic - the vision into the realm of madness. This encounter is not meant to be deadly, but instead to be very narrative heavy. Why is he like this in your world, and what purpose does he have here? That may help inform the visions your players see.
The rat minions in this encounter are intended solely to help push that mechanic along.
Other than that, keep in mind that Shatter is a near rabid beast at this point. Play him reckless, careless, and savage.
Shatter doesn't have much of a personality at this point. He has fully succumbed to madness, and does nothing but inflict pain on the world now.
Shatter was once a kind wererat. He strove to help out, and even helped around the monastery of the Black Bead monks. It was this extreme kindness that got him into this predicament in the first place, unfortunately. An alchemist asked him to test a potion out, and since then he has been unable to see the material plane, only seeing one of pure madness.
The right music helps set the tone amazingly. Shatter is an extremely chaotic and confusing encounter, so the music should reflect that. I'm quite partial to Hailstorm from Risk of Rain for that reason.