Bright Memory: Infinite

First off: you do NOT have to complete the entire game on „Hell“ difficulty! See below for more infos

Edit: According to the lovely commenters you also do NOT have to play through the game on Revenge! You can play through the game on Easy first, then beat the final chapter (as described below) on Revenge and then move on with the rest of the guide!

To unlock the highest difficulty you have to play through the entire game on the highest available difficulty. Normal enemies shouldn‘t pose too much of a challenge on this run. Liberally make use of all your special ammo types (the game usually dumps loads of it every encounter) and get somewhat familiar with the parry/deflect mechanic.

Bosses can be kited around with relative easy. Meaning you can just run backwards while unloading unto them, while using your sword to parry all ranged attacks and jumping/dodging their ground attacks.

Once you‘ve beaten the final boss you can use the chapter select in the main menu to start the last chapter on „Hell“ which WILL unlock this achievement once you‘ve beaten the final boss again.

All the strategies above work just the same, enemies and bosses just have a ton more heath now. Especially bosses get super bulletspongy this time round. But take your time, kite them while dodging their attacks and keep blasting them with all you got.

If you need more guidance on how to beat enemies and bosses, refer to the comments again. People have shared their tricks in there so give them a try!

Just play the last chapter on Hell difficulty to get this achievement. Hell difficulty must be unlocked first by finishing the game on Revenge difficulty, which you can do the same trick of just playing the last chapter on the Revenge difficulty.

Here's a video walkthrough for beating the final chapter on Hell difficulty:

Every boss can be easy killed by having The Sword Slam ability max leveled. I can’t remember its name, but which ever move that you have to jump in the air and you charge up your sword and slam it into the ground. That is the move. It does a bunch of damage and staggers. You can spam that through the entire game. Especially because the time it takes to charge up the attack your energy bar has already started to recharge.

At the very final boss you will need to give home some space at times because he will do AOEs and if you sit in the same area it will kill you. So just create space and launch your special ammo at him then rush him again when you can.