The combination I roll with are two druids and three BM hunters (so pretty much ranged), I have my tank on it's own and all my other toons stacked on the same spot making it easier to avoid stuff however if I get hit by a mechanic, I pretty much die. The highest I've done Eye of Azshara is M15 pre 7.2 while having done Eye of Azshara on M11-M12 during patch 7.2.

General advise for this guy has pretty much been stated above, everyone has their own way handling the mobility aspect of the fight. As for me, when it comes to getting Toxic Wounds, I usually just move as they spawn (most of the time, the Poison Spit syncs up with this also). My healer has a seperate round robin rejuv spam so I can heal while moving making everything a sinch. All I can say is make yourself extremely paranoid when it comes to movement, if you're standing still you're going to get wrecked.

Submerge also opens the boss up with a 10-15 second time window where the boss casts Toxic Wounds/Poison Spit; I use this time to kill the arcane add making the remainder of the phase at least manageable. After an add is dead he usually casts Rampage leaving him open yet again to go kill the other add; however I position my tank on the boss and have my toons facing the fire add to nuke down during this window. After this, the boss is pretty much a repeat of phase 1.

The second submerge is no different except you have to interrupt him off the bat so I save roar for this window to minimise damage/position myself correctly. I usually save my healing CD for this section just to cushion some of the damage. The key to this boss is making the fight mechanically manageable and movement, you won't cop damage from the boss UNLESS you're getting hit by Toxic Wounds/Poision Spit and leave the adds up too long and the damage will be overwhelming.

In terms of heroic/mythic, you will get to a point where you can just out gear everything. However if you get this boss fight without out gearing it completely it'll make it tons easier to handle during M+. Heck, nowadays I find the other bosses in EoA more annoying than Serpentrix. Unfortunately, I can't really pitch advise on how to handle this boss as a melee heavy group as I haven't run melee group this expansion. :P