...Well... I mean, everything can be destroyed. Get Strong enough, and you can break fences (LV3), then rocks (LV5), then trees and entrances (LV9), then open up new passages through blocked entrances (LV10).
While there isn't enough EXP to get to LV10 on a standard playthrough... But if you find every blocked off area, every secret room, and defeated every enemy then break every bit of terrain you find... You can get to LV10, and break open entrances and cut down trees, and close off entrances.
...I would say the only thing you can't kill are rock walls in dungeons and the UI, lol. But regardless, you can break everything else, kill NPCs, Items, and, of course, enemies.