Q:
Terrain and exploits
It’s not an exploit, or a bug. I believe ArenaNet encourages players to use the terrain to their advantage when dealing with foes. You found a high up ledge where no one can get you? Great! (or at least I think that’s how the quote goes!)
Pretty much same as wolfgang, its not an exploit nor a bug =)
I’m not sure but I think the game detects when a AI opponent is in a situation where it can no longer reach you (i.e. a melee only opponent and your up on some cliff it can’t get to) and your attacks start to fail with “obstructed” appearing.
This has happened to me a couple times and jumping down always makes me able to damage them. I imagine anything is fair in PvP though as if you can get up there they can too.
I have noticed it initially says invulnerable when shooting from above but there is always a sweet spot where you can hit them and do damage but they cant hit you, that is what made me think it may be a an exploit and your not meant to be able to hurt them.