Shreader Sharptongue is a Vermin assassin boss. Head north-west from Senji's Corner, when you get to some ramps/stairs head west, but do not step off the roofs. Go a short way north, until you see a bridge to your west, cross that bridge and you will find the boss, who will also be accompanied by a couple of Vermin groups. The boss and the Vermin groups travel in a circle, so not luring out Shreader from the circle will eventually attract huge amounts of potentially deadly Vermin.