Sinestra is the Heroic-only boss in The Bastion of Twilight, and can be fought after having killed Cho'gall. She can be a bit tricky to solo, but it should be very easy once you know how to deal with her.
During the first phase, you simply just need to DPS her down. This should be easy enough. Once she enters phase 2, she will spawn a lot of adds, and these needs to be kept alive. It's not a problem if you kill a few of them, but don't aim to kill them all. During this phase you should keep damaging Sinestra, but you will damage her mana instead of health.
Once her mana is very low, the two eggs at either sides of her will loose their shield, and these needs to be nuked down before she regenerates her mana. This should be very easy at level 110. However, some classes will have trouble killing both eggs, and if this is the case, then simply repeat phase 2, and kill the other egg when her mana is reaching zero.
During the final phase, she just needs to be nuked down again, so just attack her, and she will die once her health is gone.