Your raid will want to set up three points of attack for this encounter; one for Sevalak, one for Tempestor and one for the adds. If Sevalak and Tempestor get too close they become empowered. Sevalak also has a knockback component so tanks pulling the names will want their back to a wall.
About every 50 seconds a group of 6 adds will spawn with the name 'a static elemental'. You have 20 seconds to burn these adds down before each one casts Static Explosion inflicting massive damage and power drain. If you have trouble getting a set down you can use AoE blockers to avoid it.
Sevalak casts the Static Storm AoE which inflicts a small power drain on application, however if it is not cured within 4 seconds it will apply a second incurable det that drains 100% of power every second for 25 seconds. If a group misses one of these cures they will need cross cures when the next Storm hits as there will be now power to cure.
Sevalak has a random knockback that will hit 2-5 people about every 10 seconds. Make sure your defilers are using Hexation to reduce the chance for it to hit. There will also be a red text joust coming from Sevalak which is blockable so give your MT healers blockers so they can stay in range for heals.
Once Sevalak reaches 59% Tempestor will spawn, have the OT pick it up and pull it away into your set attack point for this mob. I would suggest timing this spawn to happen just after you have killed a wave of adds to make it more manageable.
About 10 seconds after Tempestor spawns he will cast his Static Cyclone AoE. This AoE only has about a 10 meter range so it is possible to completely avoid it. However it still must be cured because if it is not cured within in 4 second it will cast Lethal Consequences instantly killing anyone not cured. Tempestor also has a deathtouch cast on his target every 30 seconds called Storm Touch.
Burn Sevalak to 20% at which point there will be a stoneskin and you can no longer do damage to it. Turn your focus on Tempestor and start to burn it down, just remember to keep debuffs on Sevalak or it will hit your tank quite hard. Burn Tempestor down to about 15-10% and then move it over close to Sevalak. You will get a message that they are empowered but yet vulnerable. Kill Tempestor next to Sevalak to remove the stoneskin; if Tempestor is not killed next to Sevalak the stoneskin will not be removed and a second Tempestor will spawn at 100% health. After Tempestor dies finish off Sevalak and get your loot.