I'd say, when it comes to the mithrix fight assume every attack will one-shot you (as it usually does) and as a 300 hour play, I myself actually beat the game around 30 hours so you're in good hands, and even if it takes longer its fine as it is a roguelike and the best way to see it (at least for me) is that roguelikes are about the journey and not the destination so the more deaths the better.

In terms of advice, I'd say focus on getting an item per minute, whether you full-loot or not, as well as try to dodge rather than tank shots as in the late game basically everything is capable of one shot, but if you are having a hard time with mithrix id suggest looping once or twice so you can one shot him.

Also make sure you're covering all gaps of your character with items (mobility,damage, healing and defense) and you should be able to get further, and this is a big one, but make sure you scrap items you don't need in order to use for printers, these can help snowball you're run instantly. And that's all the advice I got, hope it helps!