Just finished EoAzshara this morning, finally, after banging my head against Serpentrix for a while. FYI I am running 4 Blood DK's and a Holy Priest.

I'd suggest a few things:

1) Do this encounter first. That way you don't have to deal with the "winds" and lightning effects that start annoying you after you kill 2 bosses.

2) Prior to the first Submerge, DPS as usual, save cool downs as this is the easy part. The melee rarely get Toxic Wound cast on them, almost every time my Priest gets the message. Since I run with the Priest as main, it is easy to move her around.

3) After the first submerge, I round up the melee and drive with the priest, instant heals while running to the first Blazing Hydra. DPS and move to the other Blazing Hydra. Then back to Serpentrix. I usually have to pop a defensive ability during this phase to keep up with healing. Once back on Serpentrix, once again the Priest usually gets the Toxic Wound cast on her, but run whichever character gets it to avoid the puddles.

4) After the second Submerge, I round up the melee, run to the new location for Serpentrix, ignoring the Blazing spawns. Blow all defensive and offensive cool downs, DPS race to the finish.

YMMV, and honestly, I am not great at this encounter, so take what I suggest with a shaker-full of salt.