The fight against the Icon of Sin takes place in two parts. In the first part, you must destroy each of eight armored sections of his body. The sections flash red when you shoot them if they are taking damage. The tricky part is that several heavy and some weak demons will spawn non-stop while you are fighting the Icon. So keep moving and collecting ammo. Use your sword on the heavy demons and If you have the unmaykr, use it on the Icon. Chainsaw and sword charges will respawn on the battlefield.
The second part is similar, except the Icon will move around the tower. This makes it much harder to shoot his arms and shoulders, so focus there first. Again, you must destroy 8 sections of the Icon of Sin. Keep moving and making Glory kills to stay alive.