This fight is, simply put, a DPS race. Due to the 5-minute enrage timer, the average DPS required for a DPS class on 10-man is 4.7k, while 7.2k is required on 25, assuming you only bring the minimum number of healers. With the right gear level, this is doable with a Frost spec. From personal experience, I had a 5.4k gearscore when my guild first downed this boss in 25-man, and I was above the average DPS required for the fight’s success.
Aside from the damage race concerns, there are three main mechanics to watch out for as a ranged class: Pungent Blight, Gas Spore, and Vile Gas.
Pungent Blight is a huge AoE damage spell that will do about 50k damage to you unless you are protected by Innoculation from Gas Spore explosions. Gas Spores are inflicted on 2-3 random ranged raid members at several intervals prior to Pungent Blight. Whenever you are affected by a spore explosion, you will gain a stack of a debuff that will reduce the damage of Pungent Blight by 25%, to a maximum of 3 stacks, protecting you for 75% of the damage.
The main strategy for handling Gas Spores is to have one spore explode in melee range, and one spore to explode at range. As a ranged player, you are responsible for moving to the correct spot designated by your raid leader, such that you are affected by the spore explosion. As long as you follow your raid leader’s movement instructions carefully, you will be fine.
The last mechanic to watch out for is Vile Gas. It is a random ability that Festergut uses on a member in range, which will put a DoT on that raid member, and disorients them for about 5-6 seconds. Due to the disorient effect, you may not be facing the boss when the debuff disappears. Be sure to reposition yourself as quickly as possible, and resume DPS.
Pet Management is minimal, as your Water Elemental takes negligible damage from AoE. Just set him and forget him.
Icy Veins should be saved for heroism/bloodlust. Likely, this will occur after the last wave of spores before Pungent Blight. Depending on when HeroLust is called, you can fit in an Icy Veins at the very start of the fight, and it will come off cooldown after the last spore. Save Cold Snap for this moment, so that you can snap back Icy Veins in case HeroLust is used sooner than expected, or alternatively, to stack two Icy Veins during the period.
For the most part, Deep Freeze should be used whenever it is off cooldown, and Brain Freeze should likewise be used as soon as possible (due to the gear requirements for this fight, it is assumed that the mage has at the 2-piece tier 10 bonus that grants haste whenever Brain Freeze is used). Try to anticipate when spores will be cast, so you can use these spells while moving to your designated spore location.
If Deep Freeze and Brain Freeze are not available for use when you are moving to your spore location, Ice Barrier is the ideal spell to cast while on the move. Otherwise, DPS takes priority over casting Ice Barrier in this fight. There is a window of opportunity when you are afflicted with vile gas to cast Ice Barrier as Well to minimize the damage taken while disoriented.
Ice Barrier can save you some damage if you happen to miss a spore by accident. Two stacks of inoculation plus Ice Barrier is sufficient to survive, granted that you are full health. I would advise against purposely avoiding a spore, since an improperly timed Ice Barrier, or a lack of heals may very well kill you.
Ice Block can either be used to cancel out the effect of Vile Gas, or to prevent damage from Pungent Blight, allowing you to ignore the spore mechanic altogether. Human mages will be able to use their Every Man For Himself racial to get out of Vile Gas, making Pungent Blight the ideal situation to ice block. For other races, however, choosing one over the other has their own tradeoffs.
Ice Blocking during Pungent Blight allows you to not move during spores, saving you time from running towards and away spore spots, but will leave you open to the risk of getting disoriented for up to six seconds with Vile Gas. Alternatively, Ice Blocking Vile Gas will save you up to 6 seconds worth of disorientation, but will cost you several global cooldowns spent running back and forth from the spore spots.
Ultimately, the choice will be based on raid composition and positioning. On 10-man, you will want to save Ice Block for vile gas, since you are more likely to be targeted due to the lower number of people at range compared to 25-man. On 25-man, you may be positioned significantly far from the spore point, which will make Ice Blocking for Pungent Blight the ideal decision. If you are the designated spore collapse point, you won’t have to move anyway, and are free to save Ice Block for Vile Gas.
If you decide to Ice Block during Pungent Blight, then it is likely that you will want to save Cold Snap for after you Ice Block, so you can use it again once your Hypothermia debuff wears off. Post-HeroLust, the choice of using Ice Block will depend on your raid dps. If the damage output is high enough to kill Festergut before his second Pungent Blight, it is safe to ignore the spores for the remainder of the fight, allowing you to save your second Ice Block for Vile Gas.
Blink is useful for moving to distant spore spots, as well as moving to the melee group if you are the extra spore amongst the ranged and are instructed by the raid leader to do so.
Bloodstained Surgeon’s Shoulderguards (251) – Lacks haste, which is a shame. You should be getting the tier shoulders for the 4-piece bonus, instead. Better off as a holdover until you accumulate enough Frost Emblems.
Plaguebringer’s Stained Pants (264) – This the likely the best pants in the 264 ilvl bracket, but only if you need the hit. Otherwise, the crafted 264 is better.
Plague Scientist’s Boots (264) – Easily the best 264 boots in the game, as it is the only item in that slot that has haste. Save your gold/dkp for this one.
Lingering Illness (264) – Lacks haste. The Frost Emblem belt is better. Even the 264 PvP belt is an upgrade over this, which is quite telling.