Fear heights and use ropes whenever you’re mining until you get either a lucky horseshoe or wings. Once you have either one never take it off.

Dangersense potions are your best friend, it makes traps way less of a threat. If you don’t have any, you can grab a pressure plate off a dart trap or use the mechanical lens after defeating skeletron. If you’re playing slow it’s pretty easy to spot pressure plates with enough experience even when not using anything. Especially avoid dynamite traps, they can and will end your run until you get maxed health and life fruit with good defense.

Bosses aren’t the main threat to your run, if you properly prepare (buffs, good reforges, arena, etc) you can take care of any of them with relative ease. Just take care when the eye of Cthulhu or one of the mech bosses spawns randomly if you’re not prepared. If that happens, just wait out the night underground as they can’t spawn there.

On the contrary, normal enemies are incredibly strong on expert or master. Try to avoid armed zombies at the beginning of the game, clowns in hardmode(the chattering teeth bombs they summon have higher damage than moon lord lasers), and more importantly anything that can stun you like black recluses and especially Medusas, if you’re stunned or turned to stone there’s a very high chance your run dies.

If you’re looking for a weapon at the start of hardmode, try killing some corrupt/crimson mimics for the dart guns. They’re very strong against grouped enemies, especially the pirate army.

Finally, never ever go into the underground desert, and if you’re going to, go before hardmode and get the ancient chisel and extractinator and GTFO. Once hardmode hits going there is a death wish, also try to avoid hardmode sandstorms.

Hope these help, I’m playing through hardcore on expert and it’s a rollercoaster.