Granted, this is 25 man, but the fight will be similar.

Likely only need 1 offtank for the adds, although they were hitting the person tanking them with a +200% damage debuff.

Burrow phases seem time based, not HP%. Although this might depend on permafrost.Frost has to get knocked down by ranged in order to create the permafrost, preventing him from burrowing down/adds burrowing up.

Spikes chase people when he's underground. Bid Bad Wolf from Kara with bigger teeth, but less chance of insta-death. Spikes *will* go through permafrost.