This can be a tough fight on Hard difficulty, due to the ridiculous damage her crushing vines can do. I'm assuming that by this stage in the game you know how to fight pretty well, so I'll not be covering brawling technique or combos.
Ivy has two life bars, one white, one grey.
Phase 1 - White bar:
The plant will thrust tentacles into the ground, and weeds will spring up in a line, shortly followed by choking vines which will tangle Batman if he's caught in them and cause constant damage while you bash the 'A' button to free him. If the weeds appear under your feet, double tap 'A' to evade quickly.
Keep avoiding these attacks while firing the batarang at Ivy (it will auto target, so just use the quick throw). This will do only very marginal damage, but over time may save you from repeating the tactics an extra time.
After a few choking vine attacks, Ivy will say something vaguely insulting and the plant will rise up and spawn glowing spores to throw at you. You can avoid them just by walking sideways, but you'll want to attack while the flower is open to cause heavy damage to Ivy.
The timing may take some practice, but it goes like this: When the first spore launches, take a couple of steps, throw a batarang, then evade the next spore with double-tap 'A'. Then Ivy fires a whole lot at once, which you should be able to avoid easily enough by walking aside.
Ivy then goes back to the vine attacks, and the pattern loops until her white bar is gone. Run up to the open flower and spray explosive gel on her to crack the casing.
Phase 2 - Grey bar:
This goes pretty much the same as phase one, but Ivy decides to call some help in. The game will auto-save at this point, so if you die, you'll start at the beginning of Phase 2.
Again you'll be avoiding the vines, but there will also be security guards trying to kill you. The best advice I can give is to get to them quickly and try to knock them over the edge of the platform. She seems to spawn 6, then 3, then 6, 3, 6, 3 in a repeating pattern. Again, try to throw batarangs at her when you get a chance as it will make the difference of fighting 3 sets of guards or 4. Guards can be tangled in vines as well as Batman, so use this to your advantage, but be careful with combos as you may combo yourself right into a vine cluster...
When a wave of guards has been dealt with, she again attacks you with spores, and the tactics for this are identical to the first phase.
Once her health is down, again spray her with explosive to end the fight.