Hey,

i reached the Dragonspire Dungeon and at the entry
i drop the letter in the mailbox.

Ok all fine until i saw this Dragon...

I know there is somewhere a switch, a lever or something else.
But before i reach any of this stuffs i was burned and killed
from the dragon, because he come through the path and block the way

That isn´t a friendly act of relationship

I´am ask me: I´am a lame duck or which strategie do you use ?
Run so fast you can, passes dthe dragon and searching of a lever or something
else ?

Ahh....and i have the dragon armor (i stolen it), but only one can wear the armor

Greeting Marc

You kinda have to be patient. Make sure yer front guy in the lead has the armor on and is selected. I lost about 100 hp in total till the thing shut up. The switch is on the wall at the end. You can try running past it..the narrator should say something as to how the armor is helping you. Until he does that, I don't know if you can pass the invisible wall (the bronze dragon may make it hard to walk past..sorta like when a NPC is blocking the way in a hallway).

Trial an error..might involve a bit of waiting around the corner, running up the hallway, take dmg, run back and wait a bit..heal..or whatever.