Pull out the big heals
Huge amount of damage would happen when the debuffed tank does shadow damage, and Arthas is going to begin pounding the other tank with an extra one hundred percent haste. This burst damage phase is completely telegraphed to you through the blade soul gold, so pull out the big heals once it has been cast. Your mana will be stretched thin on this fight due to the sheer amount of healing necessary, thus using any mana regeneration abilities any time when you can get them.