im always the first tank to go in for festergut and i usually pop a armor pot before I charge in. Then i cycle thru my many bubble cool downs until I get 9 stacks and the other tank taunts off. This inckudes 4pc t10, the skeleton key, shield block. Rarely I will pop last stand for extra cushion. When it comes time for me to tank him back I pop another armor pot to last me until the final Blight 3. Here is when I use Shield Wall, followed by everything else I got.
It's just a mastery of learning to balance your defensive cooldowns.