RESIDENT EVIL 3

To unlock Inferno mode you'll need to beat the game on hardcore & then nightmare mode, for the achievement you'll need to beat the game in less than 2 hours without saving more than 5 times, to make this achievement way easier you can use the infinite RPG & any defense or healing coins you buy in the shop, what I did to get all the points was keep loading old saves at certain points like the hospital zombie waves or getting the fuses & just keep killing the zombies with whatever gun you need for your next record
I would recommend using your 5th save before the last boss fight as he has an op move that can kill you very easily, the only way I could deal with him was by buying the dodge manual from the store & dodging his 3 or 5 attacks then attacking & then just rinse & repeat
Doing it this way I was able to beat the game in an hour & 2 minutes

To complete this achievement you need to first beat hardcore and then follow that up by defeating Nightmare. If you've farmed enough kills at the "defend Jill" stage for the 2000 kills and met prerequisites, you should have enough shop points for Infinite Rockets, both of the hip pouch boosts and 2 of every stat/ recovery coin.
Saves I used for my 2 runs were:
1) immediately before nemesis fight one
2) immediately before nemesis fight two
3) [this save can be wherever you wish]
4) immediately before nemesis fight three
5) immediately before nemesis fight four (final)

If you die at any stage, return to main menu and load last save to keep your save state run time. This is trial and error at times but provided you know your route through the game and what you NEED to get, (e.g. forget about weapon pick ups and ammo for example), you should clear this in less than 1 hour 30 minutes with good final nemesis luck.
My 2 times have been 5 saves 1h26 and 5 saves 1h12

God speed and be prepared for mass annoyance.

Not much of a guide perse, but a tip in relation to the final Nemesis form which can be a bit of a nightmare on this difficulty seeing as if it manages to stunlock you you're pretty much done for.

I recently played through the Z Version on Inferno using the DLC which unlocks all the unlockables including the Infinite Rocket Launcher, Coins and S.T.A.R.S Manual in order to speed through the stack quickly. The latter two can be stacked to improve their effectiveness. You can also unlock these in the Bonus Shop allowing for a stack of 4 Recovery Coins, 4 Defense Coins, and 2 Manuals etc.

Using 4 Recovery Coins, 4 Defense Coins, and 2 Manuals in tandem with the Infinite Rocket Launcher will make you practically invincible, allowing you to tank through the Nemesis' stun lock. I got trapped in the stun lock 4 times and only had to heal twice, but was able to defeat it on my first try.

Using this loadout will make this boss fight far more doable if you're willing to drop few dollars/euros/pounds on the DLC considering it can be a little cheap normally.

Just a quick PSA since no one seems to care enough to post this.

The spawns are the same as Nightmare so if you got caught off-guard by the Gamma in the sewer behind the waterfall -> rocket wala he's gone. I would recommend doing back to back - runs since each runs barely takes above an hour even if you get lost once or twice.

Pouches; the one near the Fetish-substation is gone, but getting the one in the sewers with the Battery pack and the one in the warehouse near the end are more than enough. Oh, and the 3 next to the first storagebox ofc.

Make sure the Inf-RL is your first purchase, get all coins, pouches and the manual and that's all you really need.

Just few TIPS from my experience. This is not as bad as it sounds. I have beaten it using 5 saves, 2 deaths (apparently) and 1:20 on the clock. My save points were: 1) As Jill - first opportunity after meeting everyone in subway station 2) As Jill - in the shop after grabbing key to side alley (before rolling head) 3) As Jill - after train crash 4) As Jill - after hospital siege/receiving antidote 5) As Jill - after killing Nemesis in the round arena with big tanks.

Hardest parts for me were: getting to first SUPPLY BOX/Save point, Rolling Head and final battle...

- each cutscene is a skippable by pressing "start" and X. Radio convos are skippable too - these can slow you down or disable action button for door and etc.
- you should have 6 coins, 2 pouch extensions and rocket launcher "purchased" from in-game store for this run. I unlocked mine after completing challenges but you can get DLC to unlock immediately. These apparently stack with unlocked in game for extra functionality.
- you can but don't have to have purchase pick lock and bolt cutter. Pick lock is next to the fence entrance to 4 generators area in a box. Bolt cutter is after you extinguish the fire with a hose.
- If you unlock all the pouches the only extra one for Jill will be in the sewers area after using the "battery pack" on the door upstairs. It's good to have all pouches to get as much space as possible for first aid spray/herbs for final battle - I think I got 7-8 of these items and used 5 or so. Don't go out of your way but try to accumulate the health items and drop them into chest across the run to equip on the end. Run run run and bazooka everything! If you get caught into a zombie you can potentially survive after holding A. You get full health back sporadically across the main events of the game
- GETTING TO FIRST SUPPLY BOX: do not kill anything - it's risky trying if your aim is not perfect, just run as fast as possible, cut corners where possible. Shooting "tutorial" is a skip - just quick turn (DOWN + B) and run away towards the bus/gap underneath. Elevator zombie can be tricked by running a circle around to get him away from the elevator button. I didn't even bother shooting him.
- You don't need ammo, weapon pick ups or Nemesis Cache drops - waste of time and space.
- Everything is one hit kill with rocket launcher (except Nemesis). You can just proactively shoot around while running. I don't think any zombies will resurrect after rocket launcher. 1 hit will stun Nemesis in earlier areas so sometimes is easier just to hit him outside of combat arenas and run away without looking back. After restoring power when you get to main street and there is a tempting yellow shortcut ladder, he slides down it and if you are too far, he has enough space to dodge your rocket and the moment he jumps towards you - you can be dead. I just shot the zombies on the left and run down the stairs. Most of the Nemesis (forms) boss battles are 4+ rocket hits and take seconds if your aim doesn't suck. Just spam spam spam rocket shots.
- ROLLING HEAD - skip the cutscene that plays on top of the staircase, DOWN + B to quick turn and ruuuuuun all the way to bottom and THEN right, away from head. Give it few seconds before you continue going up the stairs. I had Rocket Launcher unequipped but dont think holding it slows you down.... realism, eh?
- If you need a good guide to follow along (if forgot enemy locations and quick shortcuts) I recomend having PowerPyx's 1:10hrs long video - I play these x1.5 speed and pause when think there is something important I need to focus on.
- I actually saved my 5th time before round arena battle so each time final form got me I had to run for extra 45 seconds from save point and do arena battle (skip cutscene, 2 shots, skip cutscene, 3 shots) then get extra health item (you get new spot after losing the cure) then try final boss.
- FINAL BOSS - It's really not that bad. I struggled because forgot there is a dodge button lol (right bumper). If you get hit once - use health item. I started with far right push of "battery" then went to left side then did the right one on the end. when you are running across the room its easier to time the dodges but make sure you have camera facing the Nemesis.
- you get a pistol in final scene (magically) so don't need to grab it from last SUPPLY BOX.
- I don't think it matters if you die for final score but after each time I was going out to main menu and reloading the save file. If you "continue" the timer will add up. Inventory screen doesn't pause the timer. The settings menu does. If you make 3 saves and reload 2nd for whatever reason, get farther in the game than before and then save - That becomes your NEW 3rd save not 4th in a row.