From what I have experienced so far, Full Fending will be really good at taking tank busters from A12S. Without Grit, you are at the mercy of your healers to help you survive the tank busters reliably.

I usually drop to 20% in Grit with some CDs from the 1st and 2nd Half Gravity. Granted I just realized there is another way to cycle CDs better but having more HP makes it easier for your healers to just do damage without worrying about your survival before topping you up. Different for WAR with the slaying ring if you have DRK for A12S since technically you can have DRK tank most of the tank busters instead. But then again A12S will be a joke in i270s for the minority few, things will die so much faster that you have so much CDs to spare. You prolly will only see 1 full cycle of the last phase in full gear if the rotations don't change suddenly when you push the HP bar to a certain %, like Cruise Chaser where you can skip so many things with high DPS.