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. After an unexpected death, I scrolled through the combat notes and saw that this skill had hit for 572,684 slashing damage. If you notice he is casting this, get out of range!
Unless you have the DPS to burn down Welkus fast, during 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.
At intervals another portal will open and eventually spew out another Elemental Guardian.
The portals appear at random locations and by the time you get to a portal it has already grown in strength and is about to spit out a guardian.
If the portal is close behind you, kill it and prevent the Elemental Guardian from spawning.
Depending on gear and healing, some groups are able to completely ignore the Elemental Guardians and just burn the name.