General guide for gear progression?

[sick plank]

I've just got to the nether but defeating the gateway was so impossibly hard, that I only managed by cheesing it, and I have full cobalt armour + katana, and now i'm in the nether things are even harder. What am I missing as I thought cobalt armour ( an Armour generally considered a more hard mode armour) would do a lot better

[tardy shadow]

have you set upgrades in? defense slots matter a lot. 80% melee resistance that stacks with cobalt's natural resist(multiplicatively but still). katanas are just not very good weapons either, a manyl sword/cleaver with invar/steel for ductile is far superior. you also want to invest in some form of ranged damage and higher mobility. ars is your go too especially for bossing when many of them have telegraphed but incredibly large attacks.

armor tends to be a little hit or miss on bosses so you want a way to recover from being half a heart more than defenses to stop that from happening because it will eventually. hydra heart is a worthwhile investment

[sick plank]

I have quite a few but ill max it out and see if that makes a difference; and I'll remake my weapon to be a clever

[sick plank]

I don't have ancient debris yet, So for the cleaver would cobalt blade and invar everything else be okay?

[tardy shadow]

might as well do full invar and smack an emerald on to max out invariant dmg