One Step at a Time

There are a number of replies to this comment that make the same point, and they're right. This achievement is trivially easy if you get a friend to help you. The fastest, simplest way to complete the achievement is to get as far as you can toward the end of the quest then have a friend share it with you. Complete the quest then hearth out. Make your way carefully back to the quest giver (falling damage counts against you, as does stepping on a fire) and cash it in. Done. My comment is directed toward people who wish to do the achievement solo, particularly solo melee.

The portion of this achievement that is completed while doing The Crumbling Hall can be done by a single melee player. Avoid the statues and any patrols coming into the quest area, then as others have said, avoid the falling stones. The reddish statues in the quest area are the tricky part.

These statues only activate once you are inside melee range. For pairs of statues, sneak between them by going right down the middle. If this doesn't work for you, you can also try running very close to a statue and then immediately running away to activate it, then kiting it through boulders until it dies - this can work, but it's quite tricky to get out of range before the first melee from the statue hits you. For the statues that are close to the keys to the spirit walls, approach on the line directly between the two statues and inch closer and closer. Your gear should go gold but be unusable at first (too far away), but inching closer still lets you activate the key without turning on the statues. Back up carefully and you're good. If you do activate a statue without getting hit, you can kite it through the falling boulders until it dies.

If you get the trap room with a patrolling statue you can avoid it by going in a clockwise loop around the safe traps. (The trap that is shown on the square as you enter the Ruins is the safe one for the day.) If you get the trap room filled with statues waiting to activate, the safest path is again to go around the outer loop, staying as far away from the statues as you can without hitting a trap. Again, park yourself as far from the statue by the key without standing on a trap and open it. Go around the loop clockwise again, or sneak past directly to the artifact, and then hearth out.

The portion of the achievement that is completed while doing The Thunder Below is awful for solo melee. It's a nightmare, but it's doable. Getting to Milau without taking damage from the traps or various NPCs is bad enough for a non-stealth class, but the Milau fight is incredibly long and prone to messing up. If it's possible, get a friend to come do it with you and be done with it, but if, like me, you're stubborn about such things, then you can do this fight.

Getting to Milau involves passing through a trap room filled with lion statues that come to life based on proximity. I had my best luck going on the right side of the path through the room and jumping the last row of traps rather than trying to follow it through the entire room. Expect one or more of the lion statues you pass to animate and hit you. Kill it and hope that it gets replaced with the inanimate version, then abandon your quest and start over. It might have been chance, but I never got two animated lions in a row at a particular location (though they do seem to respawn into the animated version fairly often.) There is a second version of this room that has two Mogu statues patrolling as well as the quilen statues that can animate as you walk through. I'm not sure whether it's possible to get through the room unstealthed while they're alive without either taking damage or using an ability like heroic leap to jump the traps.

Avoid the giant cleaving statues of death in the upper portion of Milau's room. You probably didn't need to come to Wowhead to learn that part...

Milau has three primary abilities: a lightning pool, lightning breath, and a deathgrip/explosion ability. Start the fight at range so that you don't immediately get meleed. The dragon will either begin with a breath or a pool and then alternate between the two abilities. When the dragon is throwing pools, just keep running and you won't get hit. Leave an opening so that you can get back to the boss for dps. When the dragon is doing its breath attack, you can run in and get a few stabs in. You must be out of melee range before the breath attack is finished or you'll get meleed before the next set of pools begins. You get about two seconds to actually attack the boss before having to run back out, so any sort of DoT you can apply is definitely worthwhile. Playing this part safe is the key to getting it down. Don't wait for that last GCD, just get out and use it next time.

Twice during the fight (at 66% and 33%, perhaps, wasn't paying close attention) it will pull you down to the lower level and then explode. It is possible that you will be pulled into fire and take damage, so the best method to avoid this is to line of sight Milau when it begins to fly off of the upper platform. The best LoS spot for this are the two large pillars on the upper platform. I also was able to LoS using the railing where Milau normally hangs out, but that was an accident and seems prone to failing. If you do get pulled down, run from her immediately. There is about a 2s delay before you start taking fire damage from the stuff on the floor as well.

Again, it's wretched, and it takes forever. Sane people should just get a friend and be done with it.

Can confirm hearthing out after killing Milau as to not risk damage on your way out won't break this achievement.

I can confirm that your pet taking damage does NOT fail you this achievement.

I ended up having my Voidwalker sat on the Mogu statues that were of any threat and just had him tank while I killed them.