Demoman Vs Sniper:

1. Introduction
All us demomen know snipers are wimpy weaklings who are horrible at close combat. Their only purpose was to be preyed upon by all the other classes. Well, maybe a sniper would have worded that differently. But a demoman is useless against a good sniper at distance. But at close the demo easily wins, often only one grenade direct hit will kill the sniper.

2. Strategy
Its simple. Dodge sniper shots. If you're still alive after running, go kill the snipers. If they are on the deck and you just ran across, throw a grenade on it. Often you will kill the sniper.