WoWDB

When she splits into her mirror images (at 60% and 30%), carefully look through all her clones before attacking them. 

Find the one that has black long hair, black headband, black shoulders, and black chestpiece.  The "black" on her armor actually has a little bit of silver to it, so make sure you examine her portrait pre-fight so you know exactly what she looks like.  All of the fakes have something "off" about them - white hair, short hair, red on the headband, or white/colours on the chest and shoulders.  It can be hard to see while they are spinning, which is why you're going off the portrait images.  Some of them look really close but there will only be one "right" Jandice.  Once you've found the right one, nuke it down. 

Repeat again at 30%.