This is a story related achievement and cannot be missed.
Defeating Poison Ivy is more tedious than hard, while attacking her keep an eye on the ground for clumps of grass. As soon as you see these clumps forming evade by them by jumping to either side. Keep tapping the to throw the Batarang, remember not to aim but use quick throws. Half way through the battle Ivy will start to spawn guards, pay less attention to them and concentrate on Ivy. Keep throwing the Batarang and dodge the vines that come up from the floor.
This is more trial an error but once you get half way through you will start from that save point.