Hurt me plenty, lol. That was my favorite level back then, today it's Ultra Violence. Nightmare level is too heavy. Couldn't go further than level 2 without cheats. It's like 10 difficulty levels more than Ultra Violence.
There is not a single cheat for everything is there? I remember IDDQD (invincibility) and IDKFA (give all guns, keys, armor, refresh ammo). Yeah the railgun is something for snipers.

A skulltag bonus gun. The plasma gun is awesome too. Those plasma balls are fast as hell.
If it's too dark maybe change the brightness. It is darker than Doom 1, but not THAT dark, maybe only in a few levels.
As for the multiplayer thing: They're using mods. Talking about mods, have a go at this:
http://www.doomworld.com/idgames/index.php?id=13852This adds lots of weapons to mess around with.
Download the WAD file, run it using skulltag. Note that the original DOOM2.WAD needs to be in the same folder. Also note that you cannot run 2 or more WADs at the same time. It might suck on one hand but it might be helpful on the other hand, especially in case you downloaded a mod you don't like, or simply want to switch back to the original Doom by just running the original Doom2.WAD.