get the boar to charge at you then run back and take the right hand ledge, you can use plummeting strike on it, only takes a couple of hits to kill it
i've not got a bow or crossbow so haven't had a go at killing the dragon but i've seen plenty of messages under the bridge saying the tail is it's weakness so presume it's possible to either kill it or get it to fuck off somewhere else
i knew about the knight up those stairs but didn't want to spoil it for you
first time i got there i had a look round the corner first where the boar is then decided to go up the stairs, saw the knight so tried legging it back down but one of the guards had followed me up the stairs so got caught between them