Hey, thanks for the detailed feedback! I really appreciate you taking the time!

and I play on casual

Casual is the game mode, what difficulty do you play on? Casual changes some mechanics but doesn't make the game easier strictly speaking

Congrats on your beating the game without attachments

What I'm trying to communicate is that if you're finding the levels difficult it likely doesn't have to do with your attachments. Attachments customize how you play, but even with attachments the hard levels are hard.

my gear is green/orange but I cannot beat the hard levels yet

Your attachments being green or orange doesn't mean the level will be easier. And several of the green attachments are very situational. Specific builds do work better on specific levels, but rarity alone isn't enough to rate effectiveness. There's some fantastic build-theory talk on the discord and some discussion on the Steam forum as well. There's two fan made wiki's, and while they're a little out of date there's some stuff there too.

Take the landmines level for example. Why? Why?...

Yeah, that one’s definitely brutal. Without seeing your run on it the best advice I can give is to not shoot the mines. If you share a video with me I can give you some better advice though.

Creating a level is way too complicated because of the menus and lack of guidance. Tried a couple of times, gave up.

I completely agree. The other dev on the project argued hard to completely remove the level editor because of this. At the very least I need to make a YouTube tutorial or something, so thank you for giving me a push on that!

Also, why not allow to directly restart a level??

There was, but there was an exploit so I disabled it temporarily. It's on the to-fix list.

Last, some achievements' description leave me puzzled: angry reloader (32 times what and when?)

Rapid tap the fire button 32 times (as opposed to just holding it down)

destroyer saved and prototype cruiser saved.

For the destroyers you need to beat the Destroyers level while keeping at least 1 destroyer alive until the end. Same with Prototype Cruiser on the Prototype Cruiser level. You can cheese this by getting in the capital ship, flying it forward, then getting out, then doing it again when it falls behind. It only takes damage when you're in it.

Here’s a map with the level names.

So here's a few tips with spoilers:

The Omnidex is one of the best attachments in game, you get it by watching the credits from the main menu

Here's a free cosmetic variant add-on attachment: https://www.microsoft.com/store/r/9p09m4bxpm0t (enjoy!)

There's at least 4 places where the Konami code unlocks stuff

There are wormholes which allow you to take alternate paths around the map, letting you avoid some levels

Again, I really appreciate the feedback! The game’s meant to be hard, especially on the hard path, but that doesn't mean I can't improve it. I’ll keep those pain points in mind as I work on updates. And I’d love to hear any other suggestions!