Doom has many great weapons to choose from, ranging from assault rifles to the ever-glorious BFG, but there's nothing wrong with using a Super Shotgun to shred your opponents into meatloaf.
Find the Super Shotgun in Doom
First off, make sure you get to level four, which is the Argent Facility. Once you're here, you'll be asked to take care of the cooling area, as part of shutting down the Argent Tower in general. Once that's done, you can begin hunting down the Super Shotgun.
You'll find a Summoner standing in your way, so make sure you fill him with lead so that there's nothing left of him, using your other weapons. Once that's down, make your way outside, and follow the steps on the opposite side of the doorway. This will take you along a rocky path, but you're going the right way – at the end of it is the Super Shotgun.
Use the Super Shotgun in Doom
You'll find that the Super Shotgun has a much bigger kick to it than the regular shotgun, but it can take a little more time to reload. In that scenario, it's best to save it for some of the bigger enemies in the game, like the ones that come running at you.
It's also not much for distance, so try using your pistol and assault rifle for enemies that are lobbing stuff at you from afar, while saving the Super Shotgun for when it counts against enemies right in your face. Good luck!