Here is a guide I wrote for anyone that is on the PTR right now I found it quite useful to get in and complete the first boss along with getting the achievement.
Hello and welcome to the High Priest Venoxis 5 Man Heroic Encounter Located in Zul Gurub. My name is hags and I will be telling you
everything you need to know to defeat this boss.
Venoxis starts out the fight up on his platform surrounded by Poison clouds.
Your tank will want to start the fight by pulling him out into the middle of the room.
Venoxis will begin in his Troll form with some pretty simple abilities.
His first ability is his Whispers of Hethiss which is a Poison Bolt shot on random players which can easily be interupted with its long channel time.
His second ability is Toxic Link which is cast on two random members. As soon as you see the link you need to get far apart untill the link breaks
for minimal damage this is not interuptable.
At 75% he casts his transformation called Blessing of The Snake God where he turns into his snake body. During this phase it starts
to get a bit more tricky and you really have to pay attention to your surroundings.
His first abililty as a Snake God is he creates a Labrinyth type maze on the ground out of poison that you can not stand in or jump over,
you must move through the maze to tank and dps the boss.
He also periodicaly spawns poison puddles underneath him so your tank must pull him out
of them and your melee must move around them to avoid being stuck in the maze.
His second ability is also a 1 shotter which is Breath of Hethiss which is a 3 second channel that can not be interupted.
No one can be in front of the boss at any point while he is casting this including your tanks or it will instantly kill them.
At 50% Venoxis goes back to the stage and begins to cast Bloodvenom which is a stream of poison much like Kologarns Focused Eyebeam ability from
Ulduar. You must avoid this at all costs while staying inside the maze. This ability lasts about 15 seconds before he becomes Exhausted which gives
you about 10 seconds of free dps to help burn him, an ideal time for Heroism/Time Warp.
At 25% he takes on his Snake God form once again and you begin the maze process over again untill about 10% when he goes back to the platform
and begins to cast Bloodvenom. He then goes back to Exhausted and thats it he should be dead soon after.
This concludes High Priest Venoxis abilities. Good luck have fun.
You can watch this encounter on YouTube just go to "Hagsism" Channel.