So not only did I win, but I got to the secret ending and beat THAT too!
At the end of Hell, in the city of Dis is the Spider Mastermind. Killing him means you end the game and win. However, if you are invincible upon entering Dis, and happen to be carrying around a thermonuclear bomb (I was), you can nuke the level. Doing so blows up all the walls, and reveals a SECRET STAIRCASE.
Before I reveal what was at the bottom of the stairs, let me talk about my loadout: I had gone along the rapid-fire weapons route, and by this point had completely ditched my shotgun and chaingun, and was only using energy weapons and rockets for my missile launcher. In the name of energy weapons, I was carrying around a BFG 9000 (exactly what it sounds like), a Railgun (does 8d8 damage and punches through guys to damage all targets behind them as well), a high-powered nuclear plasma rifle (generates it's own ammo), and a Hyperblaster (high accuraccy and RoF). I also had 420 energy cells, and a staff that let me teleport, so I was ready to face the final evil...
John Carmack! After nuking the mastermind, the next level contains John Carmack, creator of doom. He only (only) has a rocket launcher for himself, but as soon as he takes damage he surrounds himself with barons of hell and revenants and all kinds of horrible things. I wailed on him with the BFG and railgun as much as I could before teleporting to the other side of the map and blasting railgun slugs and rockets on the other side of the room I couldn't see, and eventually one of the rockets happened to catch and kill Carmack in the explosion. FULL VICTORY MOTHERFUCKERS!