So I was testing out the latest beta release candidate to see how the new experience awarding system works, and I accidentally won the game.  Erm...  can we call this a successful test?  :P
Seriously, though, I've tried AoMr many times unsuccessfully with a technician, because (in theory) I prefer the Headshot trait over Gunslinger, because it's more ammo efficient.  Then I decided to try out a Marine game with Gunslinger and won on my first try.  :P  I was able to mitigate the ammo inefficiency from all the extra bullets fired by Gunslinger with Army Surplus trait in the early game, and with the "Efficient" bulk mod bonus in the late game.  (I'm also not a fan of the fact that you fire more slowly with the guns until you reach Gunslinger level 3, but fortunately, that wasn't a problem for long)
I didn't get Conqueror, but not for lack of trying.  I'm convinced now that the key to getting the game to consider a level "finished" is to have all enemies dead at the same time, even if some revive afterward.  I was able to achieve that on all the special levels except Dark Cathedral, where I very nearly ran myself out of bullets.  (I completely exhausted my 7.62 rounds on that level and was down to my last stack of .44, and I was only hanging on because of the above-mentioned "efficient" bonus)  With all the enemies spawning so far apart, there was no way I could get all of them dead simultaneously, not even if I had enough bullets.  (At least I was able to get the rewards, including the "don't let the door hit your ass on the way out" message)  :P  It didn't help that on Shadow Halls level 1, there were zero supply boxes of any kind, (bug or feature?  It was seed 9765) and it was an Infestation level full of Archreavers, so I was already low on ammo going into Dark Cathedral.
I had a stack of multitools going into Limbo, and I pumped them into the tech station there to fill out my remaining mod slots that I hadn't filled yet, and I put the Frenzy bonus on everything, and shortly after regretted that I didn't get it sooner, because once it gets going, it dramatically increases the damage, though that might be partially a reflection of my HR 3 plus powered armour with metabolic boost, which made me super speedy, which let me charge headlong into extra groups before the Frenzy bonus wore off.  In any case, between that and the 20% bonus from the Pistol Booster amp, and the 25% crit enhancer in the helmet and all my boosts to critical chances, I was able to gib things like Siege Ravagers and Toxic Reavers in one volley!  By the time I got to the Summoner, I was able to shred him pretty quickly, three volleys per each of his three stages.
It's probably also worth noting that this was my highest ever score in this game.  Yay!  :D
Also notable, this brings me up to General rank, and the next available rank is "God Hand" for 15 Diamonds.  With 8 Diamond badges to go, that'll take a while.  :P
Tormuse, level 20 Marine, defeated
The Summoner against all odds.
He survived for 12853 turns.
The run time was 6h 45m 13s.
He scored 9752 points.
He opposed the NIGHTMARE!
He was an Angel of Marksmanship!
CALLISTO L3 -> Mimir Habitat L1
Mimir Habitat L3 - Low Power
Mimir Habitat L3 -> CalSec Central
CalSec Central - cleared
EUROPA L2 - Secure Vault
Europa Concourse -> Asterius Habitat L1
Asterius Habitat L2 -> Asterius Breach
Asterius Breach - cleared
Io Nexus -> Shadow Halls L1
Shadow Halls L1 - Infestation
Shadow Halls L2 -> Dark Cathedral
BEYOND L2 - Exalted Summons
BEYOND L2 -> Limbo
Limbo - cleared
Awards
  CRI Star (bronze cluster) (+25)
   * 25+ kills without taking damage
  Explorer Ribbon (+50)
   * Visited all encountered special levels
  Marksman Platinum Badge
   * Complete AoMr on Nightmare!
  Marksman Diamond Badge
   * Complete AoMr on N! master-less
He killed 1101 out of 1254 enemies.
  94 archreavers
  7 plasma ravagers
  30 exalted ravagers
  22 fiends
  3 corrupted grunts
  47 exalted fiends
  5 military bots
  25 turrets
  20 rocket turrets
  1 Summoner
  6 military sentrys
  1 former CRI grunt
  19 security sentrys
  181 exalted reavers
  9 CalSec bots
  5 former CRI soldiers
  7 corrupted sergeants
  37 exalted kerbeross
  1 hellish sergeant
  7 ravagers
  22 former soldiers
  12 combat drones
  30 former guards
  7 siege ravagers
  7 fire fiends
  1 former heavy
  129 exalted soldiers
  8 former sergeants
  20 CRI marines
  6 corrupted heavys
  19 cryobeross
  3 hellish soldiers
  4 former CRI commandos
  39 corrupted soldiers
  10 ice fiends
  46 toxic reavers
  34 former grunts
  4 former CRI heavys
  26 armored ravagers
  3 former CRI sergeants
  13 CRI bots
  9 reavers
  5 toxic fiends
  21 corrupted commandos
  11 security drones
  8 cybeross
  33 cryoreavers
  4 kerbeross
  18 CalSec sentrys
  10 former commandos
  8 security bots
  11 toxibeross
Traits
  Ironman L2
  Hellrunner L3
  Son of a Gun L3
  Tough as Nails L1
  Army Surplus L3
  Cover Master L1
  Gunslinger L3
  Whizkid L3
Trait order
  SoG->SoG->Gsl->Gsl->Gsl->SoG->Arm->Whk->
  Whk->Arm->Arm->Whk->Hr->Hr->TaN->Hun->
  Hr->Iro->Iro
Equipment
  Slot #1 : AV2 .44 long revolver PBA
   * Efficient
   * Calibrated 1
   * Frenzy 3
   * Zombiebane
   * Autoloader
  Slot #2 : AV2 .44 long revolver PBA
   * Frenzy 3
   * Efficient
   * Calibrated 1
   * Critical 40
   * Hunter 6
  Slot #3 : AV2 7.62 sidearm PBE
   * Frenzy 3
   * Efficient
   * Disruptive 3
   * Critical 40
   * Calibrated 2
  Body    : AV3 marine armor PA2
   * Swift
   * Metabolic boost
   * Carrier
   * Painkiller
   * Powered
  Head    : AV1 basic helmet P2B2
   * Crit Enhancer 25
   * Tech Monitor
   * Durable
   * Danger Monitor
   * Health Monitor
  Utility : AV3 pistol AMP
   * Pistol accelerator
   * Pistol booster
Inventory
  7.62 ammo (x100)
  7.62 ammo (x80)
  .44 ammo (x50)
  .44 ammo (x50)
  .44 ammo (x45)
  gas grenade (x1)
  krak grenade (x3)
  smoke grenade (x3)
  CRI phase kit (x2)
  multitool (x1)
  military stimpack
  large medkit
  large medkit