I'd maneuver myself into one corner just before Ivy starts shooting the poisonous balls, then start running to the other side. Some quirk of the firing/targeting rate and Batman's running speed make it synch up perfectly so that you can just run from one side of the screen to another without dodging or anything, and every single ball will miss you. Once the plant opens up and I'm sure a ball won't hit me if I stop, I quick-batarang Ivy, dodge the rapid-fire poison balls in the direction I was already running toward, and repeat from the beginning. I beat Ivy with almost full health on normal, and the little health I did lose came from having to fight the guards, which sometimes threw my timing off on anticipating her pattern.