A Curious Fellow said:
Two exceptions that both think outside the box:
This guy [http://images.wikia.com/typemoon/images/a/ab/ShikiTohno_mbaa.png] looks like a total wuss. And he is... unless he's not. [http://danbooru.donmai.us/data/d454bf7426ba1cd4e6cc132c6004ccf0.jpg]
He can kill anything that is possible to kill since he has eyes that can see special lines and dots on things. If he stabs the lines, things will fall apart there, if he stabs a dot, its existence will be obliterated. This can range from cleanly slicing a chair to stabbing a person so it hits
the poison in their veins and destroys it.
So should he get a hit, he can kill anything unless it's immortal by some rule. Since he eradicates his target's existence, revival doesn't work either.
Alternatively, any character that summons/fights with water. Remember why Mercer can't escape New York (unless he travels by helicopter)?
Alternatively alternatively, get some bloodtox and supercancer, improve it up to eleven and go nuts. Or drop a nuke, works against anything alive.
P.S.
Mercer does have some moral values, that's part of his whole character. He cares for his sister and he hates the original Mercer for what he's done, despite the fact without his actions he never would have been born. So in a way he is an unlikely hero that cares about the city at large. (As far as I know) we don't even have canonical information of how many and which people he consumed, so maybe unlike we players he didn't kill thousands of civilians purely by accident.