I've just done this achievement and it took me a good amount of attempts, the attack you're describing is avoidable but I think you need to have your double jump available. The attack is made up of two waves, if you double jump away from the first group you'll just about manage to clear them as they go under you and because you was in the air, the second group will now attack at a higher angle which allows you to crouch-walk under them. If you have used Hellebore and cannot double jump, I believe the best you can do is evade with down+A but you'll still get hit at least once (there may be a method of avoiding the groups without a double jump but like you, I can't see the safe spot anywhere).
Because of this attack, I found that the best way of fighting the third form is to only use Hellebore on the boss' shield when she is already performing one of the other avoidable attacks. It's a slower fight as sometimes you won't have the time to commit to a safe attack, but at least you will be able to dodge everything. You only need to break her shield three times to win which isn't too bad, a lot of the times I failed because I wanted to attack during her move but left it too late, so Hellebore connected but didn't have enough time to recover before the boss used the gang-attack on me.
Edit: just did it again on new game plus (which I assume is Normal difficulty) and did manage to avoid it without a double jump. It becomes three waves instead of two so I'm not sure if the waves become thinner because of this and thus easier to avoid, but I'd assume not as it would be weird that it gets easier on a harder difficulty.
Last edited by Didi Mau; 04-18-2013 at 08:14 PM.