To solo, just draw witches out, move to center of bridge, wait for adds to shift, should be able to kill the witches and all adds on the left side(left side if your facing spawn) before her song is prepared. The shriekers dont even show up until her song is prepared, kill shriekers then go back to left side and kill the two sword knights and attack the witch. At that point the only adds you need to worry about are thralls.