First time I went to DD, we wiped on Deepfang 10+ times, haha.
Since then, I've gone 4+ more times, and never had an issue. So, I'll share what we typically do.
Fight the battle normally, and keep an eye out for the warning to who will be cocooned. The second they are cocooned, ALL DPS swaps to the cocoon to get them out as soon as possible. When the little spiders spawn, the group huddles so they can be AoE killed.
Repeat as necessary.
I've heard people say that hiding the cleric behind something will keep them from getting encased, but that didn't work for us, so we just fight it out like I mentioned above.
What level were you guys?