Enter Temple of Veeshan: Halls of the Betrayer at at the West Portal in the Temple of Veeshan.
There is a 365% CC Debuff (Lesser Draconic Affliction) so you will need 465% Crit Chance in order to crit on every attack. This is important, because even the trash mobs have tons of hit-points.
Throughout the zone, you will find groups of two or three Aerakyn. If there is a deacon among them, kill him first! He heals (Kite them, they can't heal if you keep moving him) You can also interrupt the deacon or mezz/CC the group as a coercer. Some of the aerakyn taunt, causing you to lose or change targets.
You can fly in the instance, and will be required to use a flying mount of some type to travel to various areas.
Be very cautious about flying into a new room, because there are sharpshooters that will shoot you out of the air and you might fall to your death.
Clear the trash in the room and avoid Captain Stinas, who roams the area flying.
Kill Captain Stinas:
During the fight you will see a message Captain Stinas prepares to unleash a devastating slash! and in your chat window you will see Captain Stinas say Feel the slice of my blade, as it cuts you in two!
This is an uninterruptable, range-based AoE, so slow/root him and joust out at least 15m away. Make sure you pull pets and mercs off as well or they are toast.
Occassionally Welkus casts A Dreadful Pounce, can hit for over 500k. You'll see a red circle on the floor when this is about to hit. Joust out of range.
Unless you have the DPS to burn down Welkus fast, elemental portals will spawn during the fight. Portals spawn randomly at each end of the room throughout the fight.
You will see a message A strange portal appears', then The strange portal has grown in strength! Eventually, you will see the message The strange portal has gathered enough strength to call forth an elemental guardian warning you of an add, An Elemental Guardian. Kill the portal to prevent the Elemental Guardian from spawning.
You can probably handle one add, maybe two. Unless you're very well geared, three will wipe you. Portals tend to alternate ends, but not always! By the time you run from one end to the other, the add will spawn. If you can't beat Welkus down before the adds overwhelm you, you won't be able to kill all the portals before adds come either. Stay at one end, kill any portals that spawn there and burn the named.