poison the water supply? Most US cities are well protected against this since there are already protocals against accidental water contamination (post 9/11 they adjusted them for harmful contamination).
chemicals - this is a pretty broad aspect, and many are not defendable so I won't go into them.
tsunami - yea there are a few areas of the world where you could create a tsunami and possibly wipeout a city.
removing key targets - taking out key officials, businesses, etc can destroy just about any medium sized town. another way is to get needed members of the community sick (various methods).
cancer - unlike poisons and chemicals cancer causing agents are often harder to detect and corelate to the victim. Of course they can take much much longer to activate.Â
it is
He could probably do a good job dsdtroying a city 
 I would run if I saw him...  :P
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