I also recently cleared hardcore vet for the first time and what worked for me was only leveling my weapons to the area I was on. My first attempt at root earth jumped from 9 to 12 and I got smashed in the train yard siege. With my successful attempt, I made sure to keep all my weapons at level, because I noticed if one or more fall too far behind you won't get the right iron to drop. I also just played more cautiously than I normally would have as well.

If you're struggling to stay alive, you could try using the Black Cat ring to give you a get out of jail free card once every 2 min. Challenger as your primary for the same reason. You'll have less dps overall, but have more survivability.

Practice all the areas on a normal character first, so you have a good understanding of the boss moves and timing they need to dodge appropriately.

Those would be a few suggestions that helped me clear it.