As for archdruid, I'm more worried by the moon druid taking a bonus action to wildshape in an earth elemental (126 hp, B/P/S resistance), cast a spell, taking damage on enemy's turn, wildshaping into another earth elemental as bonus action (thus renewing all hp) cast a spell, taking damage on enemy's turn, wildshaping again as bonus action...
In effect, the druid ignore the first 126 points of damage (or up to 252 damage if you factor-in resistance to B/S/P damage) each round with spending its bonus action as the only opportunity cost (that and can't cast spells with expensive spell components). Between being able to do that once per day and each round without limits, that is a whole lot of "free" hp. It becomes a 10th level feature but still...