There are two variables, oneshot_strength and oneshot_weakness. oneshot_strength controls how much damage you deal to monsters, and oneshot_weakness controls how much damage they do to you.
oneshot_strength values:
- Normal: 1x damage done
- Permastrength: 2x damage done
- Beatdown: 3x damage done
- Permaquad: 4x damage done
- Curbstomp: 5x damage done
- Rampage: 10x damage done
- Surefire: 100x damage done
- Bloodbath: 10000x damage done
- Normal: 1x damage taken
- Anti-resist: 2x damage taken
- Fragile: 3x damage taken
- Weakness: 4x damage taken
- Kamikaze: 5x damage taken
- Martyr: 10x damage taken
- Lemming: 800x damage taken
- Genocide: 10000x damage done, 1x damage taken
- Massacre: 100x damage done, 1x damage taken.
- Curbstomp: 10x damage done, 1x damage taken.
- Permastrength: 2x damage done, 1x damage taken.
- Normal (default): 1x damage done, 1x damage taken.
- Pillage: 10000x damage done, 2x damage taken
- Slaughter: 100x damage done, 2x damage taken.
- Beatdown: 10x damage done, 2x damage taken.
- Doubler: 2x damage done, 2x damage taken.
- Squishiness: 1x damage done, 2x damage taken.
- Extermination: 10000x damage done, 10x damage taken.
- Minefield: 100x damage done, 10x damage taken. (yes the name sucks, get over it)
- Fragility: 10x damage done, 10x damage taken.
- Realistic: 2x damage done, 10x damage taken.
- Weakness: 1x damage done, 10x damage taken.
- Sudden Death: 10000x damage done, 800x damage taken.
- Death Run: 100x damage done, 800x damage taken.
- Kamikaze: 10x damage done, 800x damage taken.
- Martyr: 2x damage done, 800x damage taken.
- Kaizo: 1x damage done, 800x damage taken.
I made this in an hour or two, so enjoy.
There is also a gambler mode. Set oneshot_gambler to 1 (and oneshot_gamblerdie to a value above 1, although it doesn't error-check), and every tic, a die with ${oneshot_gamblerdie} sides will be rolled. If it rolls 1, you will take 10000 times the normal (as in, before oneshot applies) amount of damage that tic. Gambler indeed.
And naturally, this mod massively accentuates the random damage done by monsters and weapons; a zombieman can do 30, 60, 90, 120, or 150 damage in one shot with 10x damage taken! For those of you who hate that, set oneshot_norandom to 1, and watch as that reason to cry bullshit evaporates.
Also, you can see what skill the game is currently at, even if you're spectating. Use oneshot_hidehud to (dur) hide it. Use oneshot_hud_x and oneshot_hud_y to move it around.

v7.3 (zandronum 2.0): http://www.[bad site]/download?file= ... 3-z2.0.pk3
v7.1: http://ijontichy.lostsig.com/wads/oneshot7_1.pk3
v7: http://ijontichy.lostsig.com/wads/oneshot7.pk3
v6.3: http://ijontichy.lostsig.com/wads/oneshot6_4.pk3
v6.3: http://ijontichy.lostsig.com/wads/oneshot6_3.pk3
v6.2: http://ijontichy.lostsig.com/wads/oneshot6_2.pk3
v6: http://ijontichy.lostsig.com/wads/oneshot6.pk3
v5.4: http://ijontichy.lostsig.com/wads/oneshot5_4.pk3
v5.3: http://ijontichy.lostsig.com/wads/oneshot5_3.pk3
v5.2: http://ijontichy.lostsig.com/wads/oneshot5_2.pk3
v5.1: http://ijontichy.lostsig.com/wads/oneshot5_1.pk3
v5: http://ijontichy.lostsig.com/wads/oneshot5.pk3
v4.1: http://ijontichy.lostsig.com/wads/oneshot4_1.pk3
v4: http://ijontichy.lostsig.com/wads/oneshot4.pk3
v3.1: http://ijontichy.lostsig.com/wads/oneshot3_1.pk3
v3: http://ijontichy.lostsig.com/wads/oneshot3.pk3
v2: http://ijontichy.lostsig.com/wads/oneshot2.pk3
v1: http://ijontichy.lostsig.com/wads/oneshot1.pk3