This run could not possibly have gone any better - and if it had gone any worse I could not have succeeded:
Level 1 Malek's Armor
Level 2 Laptop Boots, and just in time for the Mastermind...
Level 7 Trigun.
I think this might be about the only way to complete this challenge, as the conditions remove every possible aid to completing it. AoP blocks all powerups(which are HUGE in an AoOC game - trust me, I know - and probably even more important in a full 21 level Nightmare game). This means no invulnerability, no berserk, no computer maps, and your only way to even heal is with medkits. Marine means no Scout for stair location, and since you also can't see them from computer maps, you have to get extremely lucky with stair placement to even have a chance at this, as your best bet is to stairdive as fast as possible. It also removes the passive class speed bonus, which is immensely helpful in a stairdive. As one final fuck you, the major advantage of the Marine class, the increased powerup duration, is rendered useless because as an AoP you can't use powerups.
I did at least get the extra HP from Marine, and decided to go for HR as my first trait to try to make it to the stairs at least slightly faster. On level one I found Malek's armor equipped on a Former Sergeant who took his sweet time dying, but eventually gave me the first piece of the solution to probably one of the most difficult Diamond badges. Level 2 gave me another artifact message, and to my utter disbelief(not to mention sheer joy), it was the second piece of the Inquisitor's set. What are the odds? That level was heavy on Viles, so I was able to make the most of the Fire Immunity right away, clear the area around the stairs, and farm up some extra shotgun ammo. I continued to dive, getting extremely lucky with stair placement, and had no real dangerous moments between being able to lavawalk, ignore most enemies, and move really fast. I still had no idea how I was going to handle the Mastermind with N! level HP and no intuition(did I mention not being able to do this as a Scout really sucks?). Then I found my third(really? almost earned an Aurora Medallion in an AoOC game?) artifact in only 7 floors - and it's Trigun, giving me a final solution for the Mastermind if combat proved ineffective.
It did - I went a few rounds, and probably could have stuck it out and tried to chip its health down, but with no real way of knowing where it was until it had the chance to return fire, it was a dangerous proposition. Rather then continue to do that, not knowing if I even could eventually kill it that way, I decided not to risk a stupid death by misplay, and activated the Angel Arm to secure the partial win.
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DoomRL (0.9.9.7) roguelike post-mortem character dump
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Nathan, level 5 Arch-Vile Chaos Lt. Colonel Marine,
defeated the Mastermind at the City of Dis.
He survived 8342 turns and scored 93655 points.
He played for 14 minutes and 16 seconds.
He opposed the Nightmare!
He killed 119 out of 303 hellspawn. (39%)
He was an Angel of Purity!
He was also an Angel of Overconfidence!
-- Special levels --------------------------------------------
Levels generated : 4
Levels visited : 0
Levels completed : 0
-- Awards ----------------------------------------------------
Inquisitor Diamond Badge
-- Graveyard -------------------------------------------------
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-- Statistics ------------------------------------------------
Health 0/55 Experience 9918/5
ToHit Ranged +0 ToHit Melee +0 ToDmg Ranged +0 ToDmg Melee +0
-- Traits ----------------------------------------------------
Class : Marine
Hellrunner (Level 2)
Reloader (Level 2)
Shottyman (Level 1)
HR->HR->Rel->Rel->SM->
-- Equipment -------------------------------------------------
[a] [ Armor ] Malek's Armor [3/3] (86%)
[b] [ Weapon ] Trigun (3d6) [6/6]
[c] [ Boots ] Nyarlaptotep's Boots [6/6] (100%)
[d] [ Prepared ] nothing
-- Inventory -------------------------------------------------
[a] plasma rifle (1d8)x6 [40/40] (P1)
[b] assault shotgun (7d3) [6/6]
[c] red armor [4/4] (100%)
[d] shotgun shell (x50)
[e] shotgun shell (x50)
[f] shotgun shell (x50)
[g] shotgun shell (x37)
[h] shotgun shell (x50)
[i] shotgun shell (x50)
[j] power cell (x50)
[k] power cell (x20)
[l] power cell (x50)
[m] power cell (x50)
[n] power cell (x50)
[o] power cell (x50)
[p] large med-pack
[q] large med-pack
[r] envirosuit pack
[s] power battery (x120)
-- Resistances -----------------------------------------------
Acid - internal 0% torso 30% feet 30%
Fire - internal 0% torso 100% feet 100%
Plasma - internal 0% torso 30% feet 0%
-- Kills -----------------------------------------------------
17 former humans
25 former sergeants
10 former captains
33 imps
2 demons
15 lost souls
1 cacodemon
3 barons of hell
3 arachnotrons
2 former commandos
1 pain elemental
1 revenant
5 arch-viles
1 Spider Mastermind
-- History ---------------------------------------------------
On level 1 he found the Malek's Armor!
On level 2 he ran for his life from lava!
On level 2 he found the Nyarlaptotep's Boots!
On level 6 he stumbled into a nightmare demon cave!
He left level 6 as soon as possible.
On level 7 he found the Trigun!
Then at last he found Dis!
He activated the Angel Arm on level 8!
On level 8 he finally defeated the Mastermind.
-- Messages --------------------------------------------------
You see : floor
Fire -- Choose target...
You see : a Spider Mastermind (scratched) | floor | [ m ]ore
You are hit! You are hit! You are hit! You are hit! You are hit! Boom!
Fire -- Choose target...
You see : a Spider Mastermind (lightly wounded) | floor | [ m ]ore
You are hit! Boom! You are hit! You are hit! You are hit!
Fire -- Choose target...
You see : floor
You are hit! You are hit! You are hit! Boom!
Boom! Boom! You are hit! You are hit! Boom!
You wear/wield : a Trigun (3d6) [6/6]
Your Trigun is already loaded.
Do you want to use the dangerous Angel Arm?? [y/n]
You activate the Angel Arm! Your life is drained! Congratulations! You
defeated the Spider Mastermind! Press <Enter>...
-- General ---------------------------------------------------
1430 brave souls have ventured into Phobos:
1238 of those were killed.
8 of those were killed by something unknown.
80 didn't read the thermonuclear bomb manual.
And 75 couldn't handle the stress and committed a stupid suicide.
29 souls destroyed the Mastermind...
1 sacrificed itself for the good of mankind.
26 killed the bitch and survived.
2 showed that it can outsmart Hell itself.
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Special thanks to Icy for his ongoing thread, which helped motivate this run. Thanks also to LuckyDee and his run, further helping inspire me to pick the game back up.
Edit: After a quick look around, this also appears to be the only successful completion of this Badge on the forums(excepting papilio, who apparently has done it twice, both times without AoOC - that dude is *nuts* - and unless I overlooked one by Tormuse somewhere) so that's a first for me. Usually I only manage to copy stuff other people have done, especially for high end badges like this - I guess I ran out of stuff I could do that way =P