Hello, here is my guideline for fastest promotion tree based on difficulty of gaining each medals/badges.
1. Clear the standard game on "I'm too young to die" to raise up your skill. It's convenient to get Gatekeeper Silver on this difficulty.
2. Next, try clearing the standard game on "Hey, not too rough" with 100% kills + Clearing almost every generated levels.
Try kill Cyberdemon damageless; it's easy with dodgemaster.
Now you'll have 13 bronzes (Technician, Armorer, Heroic, Gatekeeper, UAC, Veteran, Lava, Reaper, Brick, Skull, Arena, Arachno, Scavenger)
and 3 silvers (Gatekeeper, Reaper Scavenger) and 3 golds (Gatekeeper, Reaper, Scavenger), to get Sergeant Rank.
3. Now, clear the standard game on "Hey, not too rough" with knives/fists as fast as you can with clearing Unholy Cathedral.
(15/6/4/0/0/0) -> "Warrant Officer" Rank.
4. Clear the standard game on "Hurt Me Plenty" 2 times. For once, having 100% kills. For once, 20min speedrunning.
(15/10/5/0/0/0) -> "2nd Lieutenant"
5. Clear "Angel of Pacifism" on HNTR within 10 min.
(16/11/6/0/0/0)
6. Clear "Angel of Berserk" on HMP with 100% kills (+Gargulec Medal)
(17/12/7/0/0/0) -> "1st Lieutenant"
7. Clear the standard game on "Ultra-Violence" with 100% kills + Full win + Conqueror.
(17/13/9/2/0/0)
8. Clear the "Angel of Shotgunnery" on UV.
(18/14/10/2/0/0)
9. Clear the "Angel of Confidence" on UV with 100% kills.
(19/15/10/3/0/0) -> "Major"
10. Clear the "Angel of Light Travel" on UV with melee build, as fast as you can - under 20000 turns.
(20/16/11/4/0/0)
11. Clear the "Angel of 100" on UV. It's not that hard, really. Make as many assembly as you can.
(21/17/12/5/0/0) -> "Lt. Colonel"
12. Clear the "Angel of Masochism" on Hurt Me Plenty without Badass trait.
(21/17/13/6/0/0)
13. Clear the "Angel of Marksmanship" on Ultra-Violence with 100% kills, with clearing Hell's Arena.
(22/18/15/7/0/0) -> "Colonel"
14. Clear the standard game on HMP with only knives/fists + Malicious Knives Cross.
(22/18/16/7/0/0)
15. Clear the "Angel of Max Carnage" on UV with Untouchable Medal. (It's not that hard than you expect)
(23/19/17/8/0/0)
16. Clear the standard game on HMP with only fists + 100% kills. You'll get additional Heroic Gold here.
(23/19/18/9/1/0) -> "Br. General"
17. Clear the "Angel of Berserk" on UV with 100% kills. Here, try to clear Unholy Cathedral and Vaults (do not use arena master staff, use alt-fire of Azrael's Scythe)
(23/19/18/11/2/0) -> "Mjr. General"
18. Clear the "Angel of Humanity" on HNTR as Conqueror. (To get AoHu related medals)
(23/19/19/12/2/0)
19. Up to now, you'll get Brick Silver, Gatekeeper Diamond, Techician Gold+Platinum+Diamond and Grammaton Cleric Cross if you had enough experience.
(23/20/20/13/4/0) -> "Lt. General"
20. Clear the standard game on Nightmare! under 20 min. (It's not hard :P)
(23/20/20/14/5/0) -> "General"
21. Select AoOC on N!, and clear Unholy Cathedral without Brute. (it's technically doable but boring)
(23/20/20/15/6/0)
22(option). For Medals: Get Untouchable Cross, Fallout Gold Cross, Angelic Badge -> You get Heroic Platinum here.
22. Clear Angel of Shotgunnery + Angel of Overconfidence on N! with 80+% kills. (Shottyman Platinum+Diamond+Angelic + Daredevil Diamond)
(23/20/21/17/8/1) -> "Marshal"
23. Clear "Angel of Darkness" on N! as Explorer + 80+% kills + Clearing Spider's Lair.
(24/22/22/19/9/1) -> "Chaos Major"
24. Clear the standard game on Nightmare! + 100% kills + Conqueror + Full win.
(24/22/22/21/11/3)
25. Do Angel of Confidence + Angel of Max Carnage on N! and clear Mortuary. (Reaper Diamond)
(24/22/22/21/12/3) -> "Chaos Lt. Colonel"
26. Clear "Angel of Red Alert" on UV + 100% kills.
(25/23/23/21/13/3)
27. Clear AoB+AoOC on N! + 60+% kills (Berserker Diamond)
(25/23/23/21/14/3)
28. Clear AoMr on N! (Marksman Diamond)
(25/23/23/21/15/3) -> "Chaos Br. General"
29. If you have enough experience and luck, you'll have Armorer Platinum + Gatekeeper Platinum.
(25/23/23/23/15/3)
Then your hunt for "not-extremely-hard" badges is definitely over.
To make further promotion, you must get 3 more Angelic Badges.
Quartermaster Angelic, Marksman Angelic, Lightfoot Angelic, Centurial Angelic seems doable.
Choose 3 or 4 of them and promote to "Chaos Mjr. General" (25/23/23/23/17/7)
If you want just more badges,
30. Clear "Angel of Impatience" on UV as Technician with cerberus boots to clear Lava Pits. Clear Hell Arena.
(26/25/24/24/17/7)
31. Clear "Angel of Purity" on UV.
(26/25/25/25/17/7)
32. Clear "Angel of Red Alert" two times more. For once, just on N!. For once, just on HNTR with clearing Abyssal Plains.
(26/26/25/26/17/7)
All other Angelic badges are crazy and their requirements are far beyond of humanity.
All remaining Diamond badges are extremely tough, too.
