The Poison Ivy / Titan Plant boss battle is part of the main story, so it's unmissable.
Making sure you have fully upgraded your armour, and upgraded your batarang so it causes greater damage will make this battle slightly easier.
The battle is split in to two parts.
PART 1
The first part consists of 2 or 3 sets of 2 vines rising out of the ground, followed by Ivy firing spore balls at your position.
When Ivy's vines are attacking, watch for where the grass lines appear. Standing in the middle means you can dodge (douple tap A) to either side, and while the vines are up use your batarang (tap LT) to inflict a little damage. Try and make sure you are not near the grass, as sometimes you can get caught even if you think you are clear.
When Poison Ivy fires the spore balls, make sure you have the whole width of the ground to run along. Run as she fires the first (hold A), then as she fires the second use the batarang (tap LT) to hit her opened shell, followed by a dodge (double tap A) to make sure it misses and you avoid the third and fourth. Each time you hit her and the sequence ends, she will fire several spores which you should just focus on avoiding; and running back to the side to attack again. This approach meant I didn't lose any health to the spore balls; just to those damn vines!
PART 2
Once she has lost her health the first time, and you have planted the explosives; you will have to attack her again. The game will helpfully give you back some of your health.
At this point, goons will attack you at the same time as the vines.
Try to run around so that the vines attack the goons that follow you around, and leave you in an area free of vines and goons so that you can use your batarang on Ivy. It only takes a tiny bit of her health each time, but you should be aiming to reduce her health by half in this first attack.
If you find yourself in a grassy area, use the dodge (double tap A) to jump over the goons and to safety.
Should you find yourself safe from the vines but with a couple of goons, it is better to focus on killing them and saving your health than hitting Ivy. Also, if you are waiting for a vine attack take the opportunity to attack the goons.
Remember, if you're on your own, use the batarang on Ivy!
Using this approach should mean Ivy loses around half her health by the time all the goons are killed. The spore attack sequence that follows is exactly the same as previously; so exactly the same run, throw, dodge method can be used. If you can hit her every time, you should find that when the goons start to reappear, she only takes just a couple of hits to beat.
Congrats!