Pretty much everything Psython said. The discerning factor will be: How powerful are Wynne & Morrigan?
1) the darkspawn start dribbling in from stage right and stage left when BM hits @75% health. Until then, you really only have two targets to concern you --
A) BM

the tentacles right next to BM. If you have most everybody up on the rock doing ranged attacks, the tentacles can't get to them.
2) Put Oghren right on top of BM, hammering away. [You _do_ have the Forgemasters Hammer, don't you?] Ignore the tentacles and just have Wynne heal Oghren periodically.
3) Use the ranged attacks to kill tentacles closest to BM; it reduces how often you have to Heal Oghren.
4) If you have any AoE spells like Blizzard, Inferno, and/or Tempest, drop them stage left and right where the darkspawn appear, just before BM drops below 75%.
5) Ds reinforcements appear at BM Health of 50% and 25%, so renew AoE spells just about at those times.
6) If Ds go for Oghren, use the ranged attacks to kill them ASAP. Don't let anything slow Oggie down in pounding on BM. Otherwise concentrate on killing other distant Ds and close to BM tentacles.
I'm not entirely certain, but if you get XP for killing tentacles, you might want to kill ALL of them before finishing off BM. (I am pretty certain that there is a finite number of Ds that appear, and they should most definitely ALL be killed.)