I spent an evening on this game and am missing some achievements still. I could use some insight.
"received tentacle suit" - I've purchased the body and head from vending machines, and also gotten the live version from traps. Not sure what the game wants here. I think this one is bugged - save editing AchievementSaveData.json and PlayDataSaveData.json seems to work.
"Reached perfect ending" - I apparently have to complete NG+ with less than a certain level of "fallen", but I don't know what level, and more importantly I don't know what affects "fallen" level.
"Cleared a stage with more than 4 items in the backpack" - Seems like they have to be consumable items, not equipment.
"Cleared a stage with 1 HP left" -
Strategies for this one: I used the very first stage of the game, and avoided passives that raise max HP. You will have to do a little arithmetic.
You want to minimize the effect of chance once you're down to low HP numbers.
The "active prediction system" passive lets you fight enemies and know that they won't take more HP than you expect (but maybe less) - and "reflex nerve activation" doesn't hurt.
The "Live Bait" skill is useful because you can destroy an enemy and guarantee its ambush effect - there's a drone enemy that has an ambush effect that does exactly 4 damage (do NOT take the "combat nerve enhancement" passive).
The forcefield drone does 0-1 damage when it attacks, so with a little luck it can whittle 1 HP off you so you can have an odd number of HP. Heal sprays also help with finely adjusting the HP numbers, especially if you do NOT take passives like "survival suit" that make them more effective.
You also want to open the exits ASAP so you can run away if you get one of those passive HP refresh items - normally a very welcome find in a run, but not for this cheevo! Plus it minimizes your chances to screw up once you reach 1 HP. For getting to exits quickly the "spectre system" lv15 passive is useful, and so is "shortcut algorithm" lv5.
You also want to avoid HP-restoring passives like "body relaxing agent" or "reactive armament". "pain disruptor" is obviously counterproductive too.
And you want to avoid shields unless you've reached 1 HP.