Hey i will give you my opinion as a 886 balance druid that have managed to get rank 8.
In all short if you can choose go feral cause caster have a severe disadvantage on some encounters, most notorious on the last 2 fast that can get movement intensive at some points.
For any balance druids that think that can sustain the 520k+ for epicusand 550k+ for d-ray i will give you a few tips that may help you:
For epicus not having a proper interrupt can be very anoying if you interrupt fast enough his first cast you wil have it for it's third cast also, for his second and last cast be sure to lure him to one of the corners of the rooms when he is about to cast and displacer away to outrange his cast.
For d-ray there are some thing that i can say:
First try to have enough astral power for at least 2 starsurges when he turns into the first boss (don't remember name right now) and cast them while moving away to avoid her shockwave.
For kil'rogg all try to save starsurges for him also he just makes you lose a lot of dps cause he pretty constant movement but please don't use displacer here you will need it for xulhorac.
For gorefiend pretty much the same if you can save 1 starsurge here to lanch while moving will help you with the downtime
For fel lord try to lore him to a side of the room and when he is casting be sure that you're not against a wall otherwhise you will have some downtime being knockback threw a wall.
For xul be sure to be in another corner of the room and displacer when he cast his blackhole
For tyrant just pop a health pot and barkskin.
And probably if he makes its to archimonde is a wipe he should be death b4 that or very close to so you can finish him with moonfires direct damage.
Overall those 2, are really a pain for casters and to be honest i was stuck in the last one for a while until i went into a raid and got a ED legendary drop then i had the dps to do it... it requires a lot of dps and prob a good legendary if you don't have 890+ like me
, for feral def the requirement of ilvl is lower cause they have a better toolkit to deal with most 7 rank bosses but the serpent... if you're mele at the serpent you gonna have some troubles but def not as bad as a caster on the last 2 bosses