Fighting the Twin Val'kyr
Before the fight begins, all players must use the correct portal to gain the needed essence for the battle, and prepare by standing on the right side of the room. On the pull, both tanks will head forward and position the Val'kyr, while the remaining players position themselves, healers making sure to stand behind the rest of their group.
As the fight progresses, light and dark orbs will spawn around the room. As in normal, these orbs explode when touched, doing damage in a 10-yard radius to anyone with the opposite color essence. All players except the tanks should actively seek out the closest orbs of the appropriate color. Healers should act as soakers, attempting to stop opposite color orbs from hitting the other players near them. However, soakers must also remember to keep a ten yard distance from those with an opposite color (and ranged DPS should also be aware of this when near healers.)
During the fight, the Twins will randomly debuff raid members with "Touch of Light" or "Touch of Darkness." This debuff will damage players around the debuffed player. It is recommended that players who receive this debuff run to the same color portal as their debuff, touching it to remove the debuff, and then switching back to their appropriate color and raid role. However, it is also possible to heal through the damage so long as three healers are present and players are mindful about orbs.
The rest of the fight proceeds like normal. There are four special abilities the Twins can use: Light Vortex, Dark Vortex, Shield of Lights, and Shield of Darkness. Once all four abilities are used, they will be repeated in the same order they were used first (the order changes if you wipe, however.)
For Light and Dark Vortex, all players should switch to the appropriate color of the Vortex, resume their role, and then switch back to their previous essence when the AoE is done being cast.
For Light and Dark Shields, all DPS must switch to the target being shielded. Whichever DPS are switching must first switch essences to aid in quickly bringing down the shield. Once the shield is brought down, the DPS can return to their previous essences and assignments. If DPS is low, preparing to use Heroism/Bloodlust on one shield, followed by cooldowns and speed potions on another, will help make sure the Val'kyr do not heal.
With consistency and practice, the Twin Val'kyr become one of the easiest fights in Trial of the Grand Crusader. To see the available loot for Heroic Trial of the Grand Crusader 10-man, check WoWhead's list.