This has a reputation for being one of the harder boss fights in the game (the other being Joker.) While this is certainly a difficult encounter, there are a few things to know that will make this go much easier for you.
Ivy has two main attacks: her tangle of thorns and spore projectiles. Both are easily avoided if you are paying attention. The thorns spawn in three locations - on the left and right sides of the platform as well as in the center. You will know where the thorns are spawning by where Ivy shoots her vine. This will make more sense when you see it. All you have to do is move to the part of the platform where the vines are not and avoid the thorns. She will also send in hypnotized goons to attack you. They are easy enough to fight and can be kicked off the platform if you're near a side.
Ivy's second attack is shooting poison spores. She does this in two ways: first by shooting single spores from her left and right sides and then in major, quick succession all at once. When Ivy pulls her thorns out and starts charging her spores, just move to one side of the platform and begin walking to the other side. The speed of your walk should be fast enough to avoid the spores. When the last spore has passed, tap LT to quick-throw a Baterang at her. She'll scream and begin shooting multiple spores which can easily be dodged by jumping to one side or another. Simply repeat this process and you'll finish in no time.