How to: Valiona and Theralion (Normal)
First things first, start by pulling Valiona, as you want him(her?) on the ground much rather than Theralion.
Fight consists of one dragon being on the ground, while the other one spams stuff from the air. The bosses share a healthpool.
Valiona on ground: Spread, clump up on Blackout, spread again, avoid breath
Theralion on ground: Stack, debuffed player gtfo, move from void zones
While Valiona is on the ground, the basic idea is to stay spread out at all times to minimize meteor damage and clump up when Blackout hits someone. In 10man, have everyone get to melee when someone gets hit by blackout and in 25man, you can have separate ranged & melee groups if you dont want to run around too much. Just make sure you dispel Blackout fast when people are together to avoid useless meteor damage.
Also, when breath comes, either move behind the boss OR if thats not possible, move as far away as possible from the breath, it does pathetic damage at max range.
When Theralion is about to land, some swirling looking void zone thingies spawn on the ground, just dont get hit by them and dps Valiona as long as possible.
While Theralion is on ground, its the opposite. You gotta stack up to split the meteor damage equally between everyone hit, but you must keep some players at range at all times. The easiest way to do this is to have around half of your raid in melee stacked up and the rest spread rather close to melee. When one of the people at range get marked to be hit by next meteor, simply run to melee (You have 6 seconds to react and move) and back out after meteor.
As for other abilities, if you get Engulfing Magic, if you have anyone next to you, do not cast anything (If you have dots/hots in, move even faster), as all damage and healing done will be hit as aoe around you.
If boss turns head towards you, you can preemptively move away from the void zone that will come to the ground.
When Valiona is about to land, he does a breath which will cover a third of the room (Think Felmyst). Its not a huge aoe really, so its easy to dodge, as long as you move to the correct side.
After that, rinse and repeat. The phases rotate until bosses die. Spread on Valiona, stack on Theralion.
Valiona on ground - spread out, when blackout hits someone everyone stand together., avoid breath from boss, move behind or run away. When other dragon lands avoidswirls on floor
Theralion on ground - Stand together to spread the meteor damage. 1-2 ranged need to be at range so the boss will fire meteor at them, when that hapepns stack up with ground ( like what we do with magmaw, everyone standing together and 1 ranged for adds) Engulfing magic, dont cast until its off. boss will look at you then place void zone, move preemtively. when other dragons lands she will cover 1/3rd of the room with fire, easily avoided.
rinse and repeat