Hi.
Yesterday we had a really frustating game of descent because we couldn't agree to the rules governing Ruin's movement.
What the players did was to stun (and keep it stunned) the second dragon in the stone bridge so when Ruin advanced to the point where the other dragon was, its movement was blocked and players got time to clear the dungeon freely.
I sustain that since Ruin can push heroes with his advance, it should also apply for any other thing blocking its way. Of course, 4 players beat one GM so players got away with it.