Doom: Marines and demons swapped
14A.k.a. Play as a demon against marines
So, how long could a demon last if it had to invade on it's own a military base full of marines? Wonder no further and get the answer here!
I tried E1M1 playing as a Baron of Hell using two mods:
I found a mod for Skulltag that let you play as monsters, but it didn't work on ZDoom, so I used it as a base to "port" it to make it work on any ZDoom-based port (supposedly), use the real health values for all the monsters and delete the limitations of monsters (the original mod didn't let you jump off cliffs or go through monster impassable linedefs). There's still some areas you can't reach with certain monsters because they're too big or wide. That gets even worse with specially big monsters like the Spider Mastermind or the Mancubus.
The other mod is one I created from scratch that turn every monster of the game to a marine equivalent. These marines are fast and accurate (the pistol, shotgun and chaingun marines use marine's accuracy instead of the accuracy of the original monsters), don't infight and strafe a lot, so are hard to hit. They are identified by uniform color. Every monster that had less than 100 health points were assigned 100, the ones that had more kept their original value. Because they're marines, they can jump off ledges and go places where monsters normally couldn't. They will pack and kick your fucking ass in seconds! Unless you're a Cyberdemon, then you could last a while.
There is a small bug when using both mods at the same time in which if you play as a Pain Elemental, you will fire Lost Souls, but on contact, the Lost Souls will turn into the marine version of them.
This should give you an idea of how overwhelming a marine can be for the forces of hell!
As a bonus track, you can hear an incomplete version of At Doom's Gate done by me for the Doom Remakes... Remade! project, which is somewhat near completion!