I think the action clearly builds suspense. We are initially presented with a sniper who thinks high position gives him safety and enables him to shoot without fear of retaliation, which of course he does, killing an informer and a soldier. However, when a shot suddenly comes from the roof of the house opposite, wounding him, he and we realise that he is in grave danger of his life. This suspense is heightened when we realise that the sniper will be unable to fight this duel with his rifle, thus putting him at an immense disadvantage.
The physical setting and the point of view certainly enhance the suspense and tension of the story. The rooftop setting and the feelings that the sniper is basically trapped upon that rooftop with no easy method of escape enhances the suspense. I think all of us can identify with that feeling of being a "cornered animal" in such a situation.
The point of view is also important. We get a chance to see inside the thoughts and emotions of the sniper and yet also have the benefit of seeing details of the other characters that the sniper may or may not be completely aware.
The author builds the suspense in the story through all three of the literary elements that you mention above.
The protagonist, the sniper, is lying in wait to assassinate any enemy targets. As he is doing this, the reader learns more about why he is doing what he is doing and his attitude about murdering others. As he is waiting, the tension and suspense slowly build.
The setting also adds suspense because the story is set in Dublin, Ireland, during their civil war. This was an extremely violent time. Families were split apart after taking opposite sides during the war and thousands of people were killed. People had to be careful all of the time in the city and outlying areas because of the potential for violence.
The point of view also adds to the suspense as the third-person narrator matter-of-factly describes the sniper and what he was doing as he was lying in wait.