You vastly underestimate the gains from being able to DPS through the ground phases. I'm typically almost done by the end of the first, though lately I just white hit in the air (maybe start DPSing when I'm up at 25 stacks
) and kill her solely on the ground (usually kill her during 2nd ground that way).
Besides, guardian at 0 vengeance pulls around what you'd expect from a DPS spec in DS gear at 85, and those were killing her just fine... and we're not at 0 vengeance on the ground, not by a long shot
.