Most Challenging Battle

I haven't played Throne of Bhaal in ages, so I've forgotten the Mellisan lines (most of them anyway). So here is a poll to replace it. Who do you feel in the most challenging major battle in BG2?
Major Battle= Battle with a Boss. I understand other fights (Like the Final Seal battle) are challenging, but I would still consider them a Moderate Battle, because they are not, in fact, bosses.