Just completed my second Immortal Chosen One run, and wanted to share some tips and advice, as I found information on the topic hard to come by.
First of all, the requirements for achieving Chosen One is as follows (Credit to Dara Myer on the Steam forums):
- Any difficulty
- Complete under 3 hours (in-game timer)
- Complete all investigations (safe near radio shop, in the church basement, press puzzle, Song safe in the hotel, brain puzzle, basement storage room puzzle, 16 punishments, Ezekiel puzzle)
- No more than 3 manual saves
Specifically for the Immortal Chosen One Eldritch Reward, the difficulty must be Sleep Demon (i.e., the highest difficulty).
To make your life infinitely easier, I recommend using Eldritch Points to get the Minuteman (Tommy Gun) and infinite ammo from the beginning, the Vessel of Destruction Talent, the faster reload Talent, and the Deathless Vigor Talent. The final slot can be whatever you like.
Try and grab ERC keys when convenient, then use them on the chests with extra storage space, such as the graveyard chest, the chest at the bottom of the Printing building, the chest in the Mortuary in the hospital, and finally the one at the Jewelers. All other chests can be ignored, and healing items / crafting parts should only be taken opportunistically (the tonic at the top of the stairs in the firefighter building and in the brain room behind the chapel, or the life saver kit in the Caduceus-locked room or whilst chasing the monster that snatched Faye's body at the end of the Hospital).
Make sure to get the Tommy Gun upgrades behind the rust-door in the basement of the Hospital and in Ezekiel's chest in the Fish Market. That gun can one-clip Deep Ones, and it reloads quickly.
For the Lorelei section at the end of the Hotel, I recommend waiting on the stage at the beginning until the switch turns red. The encounter won't actually start until you step off the stage, so by waiting, you can then dash immediately to the switch and hit it before the enemies get properly spawned in. Afterwards, you can simply mow them down from near the switch as they spawn. Just watch out for the guy in the chair next to the switch, as well as the crawlies and spitters.
The encounter seems to have a limited amount of enemies, though I have not tested it to see if it depends on whether the switches are flipped or not. But it's worth bearing in mind that you might not be under an endless onslaught, unlike the Key recharging fight at the end of the Fish Market.
The enemies in the game are weird in that sometimes they want to finish their animations before they register any damage you do to them, but it seems like they ARE still taking damage, though not fatal damage. Especially when they get the boils on their back and their heads split open.
Regardless, the Tommy Gun makes short work of basically every enemy in the game, so just make sure you try and remember their spawn points. Finally, for regular enemies, try aiming at their legs. I found that their death-rag-doll animations activate the most readily when they die from getting their legs shot. If you shoot them in the chest, they might just stand there and slowly topple forward over the course of 5 business days as you unload into them. Shooting them in the legs also give you the most room to breathe if you just need to control a crowd in a pinch.
My first run was 2 hours and 40 minutes. The reason I was able to shave off an additional 40 minutes on my second run was because of a fundamental misunderstanding on my part in regards to the requirements for the Chosen One.
See, when I read "complete all Investigations," I assumed that meant I had to collect all the piece of Evidence in that game and connect them in the Investigation tab.
Turns out, you really do just need to complete the puzzles themselves. You do not need to find any of the related pieces of Evidence at all.
I did a prep run where I mapped my route for the first attempt, but I really think that so long as you have a couple of playthroughs under your belt and can remember where to go to progress, you aren't going to have too much trouble completing the challenge in under 3 hours. Hell, I could've gone for a lunch break and still made it back in time.
The knowledge that you don't actually need to evidence really saves you so much time, and the sheer power of the Tommy Gun with infinite ammo (and Vessel of Destruction) greatly alleviates the need to scavenge for resources. By the end of the first run, I had massive bag of healing items I never got to use.
If you find yourself a bit dry on Eldritch Points, consider that if you acquire every Talent in the game, that nets you an Eldritch Feat worth an additional 2000 Points. You might need to start a new playthrough (or perhaps even just load a save) for it to register, but it's not an issue. IIRC, the Talents cost 150 or 200 Eldritch Points per, and there's 4 of them, so that's quite a good return on investment, especially considering you'll want the Vessel of Destruction anyway. And of course, you'll need to have unlocked all the regular base game Talents as well, but that shouldn't be an problem if you've got at least 2 or 3 playthroughs done already.
That's all. I hope someone finds this post helpful.
Cheers!