Any Savage1 cleared ASTs that can offer some insight for me?

Just had an attempt on Faust with a pick up group and while our DPS was sitting solid, we wiped twice after the tank died when I ran out of mana.

Our PLD was solo tanking it and i asked him to pop hallowed ground at around 6-7 stacks (i believe 7), he didn't and he died as I'd ran out of mana. I asked him why didn't pop his immunity and he simply said "Hallowed only at 8 stacks", is this typical of a solo tank method?

I feel we would have killed Faust if he'd popped immunity sooner, I tried my absolute best to heal. I love this job and my faith in it is severely shaken after this experience, if anyone can offer insight I'd appreciate it~ thanks.