I Saw Solis

Very Easy Achievement for Battle for Azeroth Level 120 and beyond...

1. Heroic Difficulty.

2. Clear mobs around boss area to spawn boss, engage boss.

3. Stand right in front of the boss and do not attack much.

4. Fixating birds will begin to spawn, stop attacking and just stand still for a couple minutes.

5. You'll notice a Fixate Debuff show up in the top right corner. Then two. Eventually there will be three.

6. Once you see three Fixate Debuff's show up simultaneously you're good to go.

7. Nuke Boss.

8. ?

9. Profit. Achievement Completed.

Simple explanation: Kill three birds at the same time to spawn three piles of ash in the same place. Kill a fourth bird on those three piles. Kill boss.

EDIT 2.0: To avoid confusion by the age of the rest of this comment; Everything is now fixed as of 6.2.3 and it just requires you to kill 3 birds simultaneously to spawn 3 piles of ash and then kill/let a bird hit you with a defensive on top of the 3 piles. Apparently, you still have to kill the boss to get the achievement. It's soloable now with specific classes, look through comments for guides. GL!

EDIT: Several people have mentioned that this is now fixed, and now only requires 3 piles of ash to be activated by the boss. However according to a comment here on Wowhead you still have to kill the boss for the achievement to be awarded (?)

Leaving this here as a possible strategy, but it is very OUTDATED:
As of November 16th 2014, this achievement is rather buggy and has a very vague and misleading text. Blizzard has mentioned this under their 6.0.3 Known Issues on the US forums;

Rukhran: The achievement criteria text for the "I Saw Solis" achievement is misleading - players need to detonate 5 piles of ash instead of 3, and Rukhran must be killed before the achievement is awarded.

We managed to do this today after several attempts across these last three days and got our Glory Achievement. Firstly, as stated above, you do not need 3 but 5 piles of ash to spawn you need 3 piles of ashes to blow at the same time. Secondly, the boss has to die in order to get the achievement. It is NOT done by killing a bird over 5 piles of ash. The boss NEEDS to use his ability "Summon Solar Flare" on a pile of ash and have that pile of ash activate 5 others 3 others. The ability is cast by the boss on random piles of ashes, which makes it a huge RNG pain.

The way we did it to avoid RNG was kiting the birds around the main pillar in the room. They can be slowed and easily kited. Get them to low health and just keep kiting, try to activate some Ashes aswell to minimize waiting time. Since Rukhran doesn't hit very hard, the tank can just stand there and take hits. You don't wanna do any dps since the boss will do Quills at 75% HP, forcing you to stop kiting.

The birds never fixate the tank, so have the rest of your group running together around the pillar, kiting all the birds. When you have 95% of the birds in the room and you are left with very few ashes, kite the big group of Birds over one of the remaining ashes. Then you want to do a big AoE stun on all the birds in a big group and simply nuke them down. It is worth mentioning that the Birds will not explode on 0 Health until the stun runs out. So with a stun like Shockwave, you have 4 seconds to get ALL birds in the group to 0% HP. Then you have a nice stacked group of ashes and from here on out the chance of the boss using his ability on one of the stacked ashes is pretty big. Simply wait it out and kill the boss after he spawned the birds and the achievement should pop upon the defeat of the boss.

Best of luck!