Customize enemy speed and flash move/guard duration + extra changes to make solo inferno less tedious but also more challenging. Clear ng ready dlc-free save file to unlock solo inferno immediately included.
 
Enemy speed multiplier and flash move/guard duration:

Speed of every enemy in game is multiplied, speed of player isn't.
Because I wasn't able to change those values through game files I did it with Cheat Engine.
Just run the game, load save file, alt+tab to windows and launch ''True Solo Inferno.exe'' Cheat Engine trainer file.
There You can pick speed multiplier from x1.05 up to x2.00 with x0.05 increase per choice

For flash moves/guards I've made 4 settings per each - shorter than default, even shorter than it, shortest and zero.
Sadly while zero works fine for flash moves it isn't working well for flash guards because some sort of ''extended flash guard'' mechanics of base game where even when guard timer ends the player character is still invincible for a noticeable period of time - and I wasn't able to remove it with Cheat Engine.
So after all I just recommend boosting enemy speed with x1.35 and leave flash durations alone.

Here is short showcase how inferno solo recommended x1.35 multiplier looks like:



Here is simple test footage of flash move/guard duration settings:



Changes made through game files:

- to avoid annoying medal swapping in solo mode I've added slash/strike/pierce attack attributes to every chest armor piece in game (only exception is ''Medium's Cloth'' to avoid problems in past-Dana sections),
^because this change You get 99x aura grass as reward for beating Melaiduma instead s/s/p medals which are useless now,

- added +14 skill exp gain attribute to every chest armor piece in game (''Medium's Cloth'' too) to make skill leveling pace more natural (no more dumb beehives farming required and Dana will finally level up her skills too),
^because this change ''Warrior Seal'' accessory property got changed to ''Rare Item Drop Rate +'' like ''Gambler Coin'' - Seal I and II are worse than Coin, Seal III and IV are better than Coin (Seals and Coin can't be stacked),

- changes made to consumables:
^every potion in Adol era gives the same effect now (base STR+3) so they are actually useful ''use them as soon as possible'' items on inferno - and because all potions are the same You can just exchange empty bottle for weakest earliest potion to get +3 STR asap,
^ingridients like pepper, meat, etc. don't restore HP now,
^Dana era ''Full Medicine'' doesn't restore HP now - instead it gives You ''EXP Gained ↑↑'' effect like ''Pirate's Platter'' meal from Adol era, cost to buy it from Spirit shop reduced from 25 to 10,
^drastice nerfs to fruits and meals:
*Dana era small potion heals only about 10% of HP,
*ripe fruits heals for a bit more, don't revive and grants ''EXP Gained ↑↑'' effect like ''Pirate's Platter'',
*''Seafood Quiche'' exp bonus buffed to ''Pirate's Platter'' level,
*fruits and coocked food restored HP values nerfed to the levels where on Inferno difficulty You basically get only 2 extra lifes, let me explain:
~full stack of 9 of every fruit type combined heals around 9999 HP in total (not exact value)
~full stack of 1 of every meal type combined heals around 9999 HP in total (not exact value)
^You need only 2 standard fruits for exchange to get ''ripe'' variant,
^Dana era ''Level up'', ''Strength'', ''Defense'' and ''HP'' potions costs only 250 shards now,

- equipment nerfs:
^some more broken items like flash guard/move extending items and revive talimans became ''cursed'' - removed functionality and added -255 STR/DEF penalty, some of them just got unwanted perk removed,
^removed ''Second Wind'' attribute from ''Luminous Gauntlets'' and ''Sky Crown'',
^removed ''Flash Move Time ×2'' attribute from ''Wings of Heavenly Flight'',
^greatly decreased ''Evade Rate +'' values from equipment,
^decreased speed bonus from ''Feather Boots'',

- replaced Adol era ''Former Sanctuary Crypt'' background music with ''Crossing A/A'' from Ys IX:
^''High Quality Music 2.0'' mod is required to make it work because base loop value datafile from it is a base for my music replacement - if You don't want to replace mentioned post-game dungeon music with Ys IX track just don't paste ''\text\bgmtbl.tbl'' and ''\bgm\y8_d911.ogg'' files from my mod,

''Game cleared'' save file included (slot 2 in-game) so You can start solo Inferno without completing the game for Your own - I've didn't activated any DLC so You don't need to install ''outfit unlocker'' mod or buy any DLC to use it (You can't load save file if Your DLCs set don't match to the ones from save file)
If You don't know it - You can change Your party size to solo at first crystal in village after meeting Laxia for first time.

Built on latest steam version of game.
Tested with english version of game - because japanese and french text language settings are loading parameters from different files than english one they are not fully tested but they should be fine.

Special Thanks to Melon Sc//ence (https://steamcommunity.com/id/tranceholloween) for explaining how music looping works in this game.

Installation:
1. Get ''High Quality Music 2.0'' mod by Various https://www.nexusmods.com/ysviii/mods/2 -
2. Paste and replace everything except ''source'' to main game folder (''steamapps\common\Ys VIII Lacrimosa of Dana\'')
3. *Load game, alt+tab to desktop, launch ''True Solo Inferno.exe'' and pick Your settings,

*You need to do this once per every game launch - sadly this is how external trainers works, also windows defender will flag any trainer generated by cheat engine as a virus - just add directory where it was unpacked to exclusions,

UPDATE:
Because even nexusmods flags standalone trainer as virus so to alter enemy speed and flash move/guard duration You need either:
a) download standalone trainer seen on above test video from googledrive https://drive.google.com/file/d/1j9MgX9s1RCmoopMZkipzLET3kahMw0V1/view?usp=sharing
b) install Cheat Engine software and use ''trainer.ct'' source file which was used to generate standalone trainer,

Here are Cheat Engine false-positives (which are plaguing standalone trainers but somehow not .ct files from which they were generated) explained https://www.cheatengine.org/forum/viewtopic.php?t=571857

Mod files explaination:
- \bgm\y8_d911.ogg - former sanctuary crypt background music swap,
- \savedata\ - clear new game ready save file which allows You to start fresh inferno solo playthrough instantly - just load this save and start new game without importing anything (except 3x empty bottle bonus - You can exchange them to get +9 STR bonus), none of DLCs where activated on this save so You don't need to bother with Ys VIII DLC protection,
- \script\item.scp - changed scripts of consumable effects here,
- \script\mp6569.scp - script of Melaiduma boss arena map, swapped S/S/P medals reward to x99 aura grass after boss here,
- \script\en\mp6569.scp - changed visual message of Melaiduma chest reward here,
- \script\fr\mp6569.scp and \script\ja\mp6569.scp - same as above just for different in-game text language settings,
- \text\bgmtbl.tbl - file with music loop database, changed to make former sanctuary crypt music swap loop properly,
- \text\mixitem2.tbb - lowered Dana era shop purchase and Adol era ripe fruits exchange costs here,
- \text\en\item.csv - changed equipment properties and descriptions here,
- \text\fr\item.csv and \text\ja\item.csv - same as above just for different in-game text language settings,
- \True Solo Inferno.exe - trainer to alter enemy speed and flash move/guard durations,
------------source files------------
- \source\trainer.ct - source cheat table (with my poorly written scripts xd) which was used to generate standalone trainer, use it if You have Cheat Engine software installed and don't want download standalone trainer from googledrive,
- \source\68688.lfm - saved cheat engine trainer template just before generating trainer,
- \source\pointers\ - pointers for various adresses - current enemy speed, flash move and guard current duration,
- \source\1.ico - icon image used for standalone trainer,