Destruction and I never clicked, but you get the impression that it is almost required on this fight, so I kept going between destro and demo. Demo (ilvl 505) made it happen in the end though, knowing how to play your spec. turned out better than any destro advantages on that fight, go figure.
Popping cd's and immolate aura when the puppies are about to come out gave me all the aggro and dps I needed.
To make the puppie phase less stressful - do not wait for Katrethan to start sum the puppies before you send your pitlord away, do this earlier so you have more time to get in position, a few sec's before he start summoning does a lot to calm the situation.
Track Kat's dots with a big bar on your screen, this had a huge impact for me instead of just using the spell from the pitlord on cd, it made everything less stressful to just use the spell when needed. Same with the gateway, knowing exactly when it will work instead of hanging around the area when it "should" be off cd made it easier to grasp the fight.