If you really have done
every quest in Baldur's Gate, and almost every quest in the wild, then you should be hitting the xp cap before you even enter Ulgoths Beard. You should certainly be hitting the xp cap when you come out of Durlags. Who are your party members? A lot of multi/dual class? the Ice Island should take you to the xp cap, and will only take you an hour or so to do, and then go to Werewold Island and you will be ready for "the big fight"
Happy adventuring.