Dude, use your flashlight.
Search every nook and cranny of every room
Before I got sent to Hell I had 249 shotty shells. LOL
Only reason I had so many is cuz I stopped using the shotty as soon as I started running into the chaingunners and the revenants.
That's what the chaingun is for. Mwahahaha.
Seriously though, the shotgun is easily one of the more powerful weapons. It pretty much can kill any enemy quickly at point blank range. Even the bubble gum monsters as I used to call em back in the day. I dunno what they are called now, the big dog things. They take two shells to the dome and they are dead if it's at point blank. The trick is trying to get that close without getting hit. lol
I think the only one it's not effective on is the Hell Knight. He definitely shrugs it off pretty easily. My best tip for those is make sure you have a chaingun and aim for the upperchest/head, like train in on the middle of his chest and unload all 60 rounds into him. Easiest way to kill em.
Oh and for the record I beat it on Marine, which is normal.