Relevant Talents used:
Body and Soul
Mindbender
Desperate Prayer
Divine Insight

Relevant Glyphs used:
Dark Binding
Inner Fire
Dispersion

iLvl: 497

This fight is all about surviving until Steelbreaker is dead (assuming he was chosen to be killed first). After Steelbreaker dies, the fight becomes trivial and hence this guide will not cover anything after the death of Steelbreaker.

Basically same strategy used as my 10man video, except that priests can no longer dispel the fusion punch debuff off themselves. We can still Mass Dispel, but that is rather disadvantageous for 2 reasons.

1. The total damage done per fusion punch is around 100k, thats 35k initial damage a dot ticking for 17k each. This damage can be healed back up with 2 Flash Heals. Assuming they don't crit, 2 Flash Heals would cost 34k mana. A Mass Dispel would cost 38k mana. Hence it is more mana efficient to flash heal the damage.

2. Mass Dispelling the debuff requires good timing, which is very hard to pull off and usually not worth the effort. Unless it is perfectly timed, expect to suffer at least 1 tick of damage before the dot is dispelled. Also Mass Dispel can only mitigate the dot portion of the spell (not even the full dot if 1 tick goes off) and not the initial damage portion.

Therefore Mass Dispel, which costs more mana, only mitigates 50% of the damage, whereas 2 Flash Heals, which costs less mana can heal more than 100% of the damage back.

With that out of the way, the boss casts Fusion Punch approximately every 15 seconds on average, with a rather large variance. As mentioned earlier, the 2 Flash Heals can be used to deal with each Fusion Punch but it is preferred if this method is used as a last resort, that is, when no other cooldowns are available. The cooldowns available to deal with Fusion Punch, from the best to worst, are: Mindbender, Dispersion, Vampiric Embrace and Desperate Prayer. These should be rotated so that only 1 is used per Fusion Punch. Mindbender is the best CD because it allows you to avoid not only the Fusion Punch entirely, but also the melee hits from the other 2 bosses. Note that if Mindbender were used as the CD however, make sure Steelbreaker has aggro on it BEFORE he casts Fusion Punch. If Steelbreaker changes target to Mindbender during the cast, the Fusion Punch will still land on you.

Mana is also an issue in this fight due to the intensive healing required. Therefore it is good idea to periodically, preferably during Mindbender uptime, to target Brundir and cast a VT on him for some extra mana gain. Casting VT on Molgeim is not recommended as it may cause his shield to break, increasing his melee damage by 50%.

Overall the fight isn't too difficult. It's all about staying calm and not mess up on the cooldowns, as well as not forgetting to periodically put VT on Brundir. Once Steelbreaker is down, the rest of the fight is extremely trivial, provided you have at least 5% mana. I once killed Steelbreaker with literally 0% mana remaining and my only mana option is Hymn of Hope. I would have channeled it, if it weren't for the fact that my health is on 5%ish as well...