Periodically, Shadron opens a portal to the Twilight. A team of DPS, armed with a healer and tank, should head into the portal and nuke down the add that spawns inside it. All members take 1000 Shadow damage per second while in the portal, so healing can be intensive or players can leap out as required. A shadow resistance buff from a priest is helpful here.
In a 10-man, the entire raid — except for the main tank, who stays on Shadron, and a healer to heal the main tank — should head into the portal. In 25-man raids, the number that enters the portal is not as important.
Rinse and repeat until Shadron is dead.
