Icon Sin has two phases where you have to shoot off his armor and then shoot his meaty body parts in another arena. The problem is that it spawns many large ancient demons at the same time, which means that you need to constantly understand who and when to shoot.
The general strategy is to keep moving and not to focus on “weaker enemies”. Shoot Icon of Sin shots when you are not being fired on. If you stop and spend time on some Cacodemon, something will distract your eyes and probably kill you if you play on more difficult difficulties. The best weapons that you can use are Ballista, BFG, rocket launcher and Chaingun, because they can do a lot of damage very quickly.
However, this does not mean that you should ignore your enemies. Kill demons with a chainsaw to get ammunition. Kill some of the smaller ones to get your health back. You can use your chainsaw to kill some of the biggest demons to get them out of the way quickly. The sword is perfect for Baron Hell’s fiery enemies. If they are on your way, along which you circled, it would be nice to kill them, because ignoring them will make your fight harder.
Chainsaw fuel is recovering, so the best strategy is to keep circling the battlefield (and at the same time, change the verticality level you are on), grab gas tanks and everything else in your path. Pay attention to the jumping platforms, portals, as they are a great way to throw enemies behind you if everything gets too hot. Run in a circle, constantly go from low to high altitudes and shoot at an icon of sin while driving.
Do not save your ammunition. This is the last thing you need to kill. There’s no reason to keep your BFG ammo.
If your health gets too low and you need to replenish it quickly, there will be a reinforcement at the top of the first phase arena. Take it when your health gets worse, as it will increase your health and armor, which will save you.
The second phase is actually quite simple, if you used the same strategy described above. Just do the same and bypass the boss around, get out of danger, don’t focus too much on the stern and shoot the icon of sin in those parts that you haven’t shot yet. You can tell which parts are vulnerable because they glow when you shoot them. In the middle of the arena there is also a supercharge, so don’t forget to grab it if you are on the verge of death.