My party vs. the Demogorgon was my PC (human monk w/ his +4 fists), Minsc (dualing FoA +4 and Crom Fayer), Keldorn (with Carsomyr +5), Jaheira (with Ixil's spear thing +6), and Nalia and Imoen (nalia with tansheron's bow and staff of the magi, imoen with gesen's bow and whatever +4 short sword). All had boots of speed. Minsc, the monk, Keldorn, and Jaheira were towards the middle, Imoen and Nalia at the edge.
As soon as he showed up I paused, turned on Greater Whirlwind for the monk, Minsc and Keldorn, and Whirlwind for Jaheira. Unpaused and attacked. Repeated as soon as it wore off, except Jaheira's 'cause she had just the 1 whirlwind. Jaheir somehow got confused/feared and started running around and eventually died during the battle (resurrectable with the rod of resurrection though). Repeated with Minsc, the Monk, and Keldorn again the same as the above 2 and Demogorgon went down. Nalia used wand of spell striking on the 6 armers (breach+ something else cancels spell protections) and the monk and keldorn beat on 1 while minsc drank a potion, then all 3 killed the 2nd one. And that was that, resurrect Jaheira, pick up her stuff and I was done.
How to get rid of the demogorgon
Critical Strike & Smite are far more effective against the tough opponents. Your tanks, armed with Carsomyr / The Ravager +6 / Any-Über-Weapon-you-fancy and STR 22-25 score up to 60 pts of damage with a single blow. Using their protection abilities (Hardiness, Magic Resistance) is also important.
And don't underestimate Jaheira, she still can be quite useful in ToB.
And don't underestimate Jaheira, she still can be quite useful in ToB.