The easiest map to get this on is Team Skullshot, Steel Mill (Steelworks). You only need 2 players for this.
The person getting kills should choose the blue team and the person dying should choose the red team. Have the blue player run to the red player spawn building (this is quite a long and confusing journey but once you get there you don't have to leave). On the catwalk inside the building where the red player always starts is a very quickly re-spawning sniper rifle.
If the blue player stays on that catwalk up against the nearest wall, they can see 3 of the 4 red player spawn points without the red player having to move at all. The 4th spawn requires the red player to move all the way to the left against the same wall that the blue player is up against.
The blue player should start looking for the next red player spawn position as soon as the red player's dead body vanishes. At a good pace it should only take about 30 minutes for all 100 sniper kills.