So... I assume it is intended that NPPs are required to get anywhere in Hardmode?
The Destroyer is hitting for 300s just by touching his body, and ~800 if his head touches you and the lasers flying everywhere are doing about 120... and there's just nothing left to do but XP grind to make my character any more powerful and even then, you get a whopping 1-2 defense and ~20HP out of every point you spend in VIT which isn't going to do much whatsoever unless you get yourself a couple hundred levelups or something.
What class are you going as? As a fighter, I'm doing over a hundred damage already, and I haven't fought ANY bosses yet.