Warrior tanks Porthio. Pally or SK Tanks cat. Clerics heal Warrior. Druid/Shams heal Pally/Sk. You don't want them close together because cat does a AoE Ramp up to 3k. You Split them. Kill Porth First. Then Finish cat.
Porthio will tell Nightshade to attack certain people in the raid, like "Go kill that caster/healer/melee" etc. You want to keep Nightshade and Porthio close to each other, so get a good chain agro tank on the cat and watch melee push.