Divine Insight (Twist of Fate also viable)
This boss is unlocked after defeating Cho'gall on heroic difficulty in BoT. This fight consists of 3 phases.
Simple tank and spank really. There isn't any rush here to push phase 2. The boss deals 40% less damage in phase 1 and also starts with only 60% health. You can afford to use all your defensive cooldowns in this phase since there is not much damage taken in phase 2. To avoid Wrack debuff, do not fade aggro onto Shadowfiend. When Sinestra summons small whelps, do not kill them yet.
When Sinestra reaches 30%, she will heal back to full and begins a duel with Calen (Caelestrasz). First priority is to kite the whelps to the back of the room and kill them. They will leave behind an expanding pool of energy and reanimate, so kite them out of the pool and kill them again. Once the whelps are down, attack Sinestra and she will replenish her lost health by sacrificing mana. Do this until she OOMs. By now the impervious shield surrounding the two large eggs on each side of the room disappears and the egg becomes attackable. Phase 3 starts when both of these eggs are destroyed. During phase 2, adds will also appear and they will need to be killed ASAP.
Calen will grant a 100% haste buff for 3 minutes before dying. The strategy here is the similar to phase 1, except now all of Sinestra's abilities including melee swings will now do full damage. Also whelps will now have to be killed (twice, since they leave a pool and reanimate) immediately when they appear. Since the boss is in a fixed position and cannot move, you can only leave a certain amount of pool in front of the boss before you have to move out of melee range. Once out of melee range the boss will cast a deadly attack which will cause a wipe. Thus this serves as a soft enrage for the encounter.Therefore it is preferable to finish the boss before the 4th wave of whelps come in.