My AST is so weak, why?

[Teiren]

Er, there's nothing for tanks to switch for at Ravana.

As for the different tanks, DRK will flat out need the most attention from healers, certainly, unless it's a fight heavy on magic damage. After all, PLD has a block chance for most physical damage, WAR has +25% HP, DRK has no special passive bonus except a little bit of leech through Souleater. That doesn't make DRK a total handicap because Grit is just like Shield Oath's -20% Damage Taken which is just like Defiance's +20% Healing Received & a bit of the +25% HP, but there is no passive safe buffer with DRK.