This boss is divided into 2 phases, where all the difficulties lie in the first phase. Once second phase is reached, victory is assured.
The first phase is an add control phase. There are 3 different types of waves of adds, with 2 waves of each type, hence a total of 6 waves must be dealt with in this phase. The set of 3 different types of waves consist of:
1. 10 Detonating Lashers. They cause an explosion dealing AoE damage when killed.
2. Storm Lasher, Snaplasher and Ancient Water Spirit. This trio of adds must be killed within 12 seconds.
3. Ancient Conservator. A big add that causes silencing effect unless standing next to a mushroom.
Shortly into phase 1, the boss will spawn a wave of adds chosen randomly from the set, with subsequent waves coming at 1 minute intervals (or earlier if the previous wave was defeated early). Once all three types of waves of adds from the initial set has been defeated, a second set will begin. The strategy to defeat each type of adds is as follows:
1. Ancient Conservator - Immediately dot up the add while he is casting the silence and then find a mushroom to stack on before the silence effect goes off. These mushrooms can despawn at some point and thus a new mushroom must be reached asap when this happens. Keep in mind that although the add does not hit has hard as Freya, it still hits hard enough to be on par with a typical Naxxramas boss, so zerging it without heals is definitely not an option. A suitable strategy is to keep dots up on the add while spamming heals on self. Watch out for mana.
2. Detonating Lashers - at level 85, their explosion damage upon death is not too much of a big deal. So Mind Sear is the most effective way of dealing with them. It is recommended to try and leave 1 or 2 of those alive at the end if there is time to spare so that Freya does not spawn her next wave of adds early, allowing some relief time for heals or waiting for CDs to come back up.
3. Snaplasher, Stormlasher, Ancient Water Spirit - This trio is the most annoying wave out of the set. They must go down at roughly the same time. To deal with this, start off with an AoE fear on the adds, and while they are running dot them all up. Put Devouring plague on the Snaplasher, and use a Mind Blast on the Stormlasher. Leave the Ancient water spirit alone at this point as it has the lowest maximum health out of the three. Stall for some time to allow the dots to tick by either spam heals or use Dispersion. From then onwards its just a matter of finishing off all three within 12 seconds by using Mind Blasts, SW:D, and instant Devouring Plague spams.
All this may already sound tedious for a priest, but to further add to the annoyance of this phase, Freya will occasionally summon a "Gift of Eonar" nearby which will begin to grow. Once fully grown, it will heal Freya as well as any adds that are present for 30% of their maximum health, this can spell a wipe in some situations and therefore the Gift of Eonar must be killed quickly. It only has 20k health so 1 or 2 spells will suffice.
As a priest, there are 2 or 3 cooldowns available that can help during the add phases. These are Dispersion, Shadowfiend, and if talented into, Desperate Prayer. In general, Shadowfiend should be used during the Snaplasher/Stormlasher/Water Spirit phase to occupy Freya so more time can be used to focus on downing the 3 adds rather than spamming heals. It can also be used on the Ancient Conservator phase if healing falls behind. It is not recommended to be used in the detonating Lashers phase however. Dispersion is used as a time staller and should only be saved for the Snaplasher/Stormlasher/Water Spirit phase as using it on the other 2 phases isnt as beneficial in terms of time stalling, although it should be used on the Ancient Conservator phase when moving from 1 mushroom to the next. Desperate Prayer can be used on any phase.
Once the 6 waves of adds are down, phase 2 begins. This phase is when damage to the boss will become effective and adds will no longer spawn. At this point it is pretty much a guaranteed victory.