10 Man Version
Grand Widow Faerlina has 4 adds.
After 1 minute, Faerlina enters a frenzy. Killing one of the Naxxramas Worshippers will dispel the enrage. It is recomended is to CC all the adds but one. Lower the non-crowd controlled add to 20% health before the enrage and kill them as soon as she enters the enrage. This will remove the enrage, and apply Widow's Embrace to Faerlina, and cause the enrage timer to reset. Killing an add before the enrage will also apply the debuff, but it does not reset her enrage timer. There are only 4 adds, so it is important to lower her health by at least 20% each enrage to avoid problems. It's also very important to keep the tank at full health so they can survive a hit when she enters the frenzy.
Since there is a lot of poison DOTs that she applies, shamans should use Poison Cleansing Totem. AOE healers are helpful because of the raid damage in the fight.
Another one of her abilities is Rain of Fire, it does a lot of damage, so stay out of the fire!
25 Man Version
In the 25man version of Faerlina you can use the above strategy, however, each add has over 300,000 health now. It is best to use a priest and mind control one of the adds, and CC the rest. Then the priest should take their add near Faerlina. After she enrages, the priest should use the Worshipper's sacrifice ability. It applies Widow's Embrace, and resets the enrage timer like killing an add normally does.
There are also two additional adds, that should be tanked and killed.
AOE heals, and Poison Cleansing Totem is very helpful for this fight, because of the heavy raid poison damage.
Another one of her abilities is Rain of Fire, it does a lot of damage, so stay out of the fire!