Iron Dwarf, Medium Rare

This strategy has been posted a few different times with varying degrees of clarification, so let me simplify it a bit.

Just completed the meta achievement a few minutes ago on an ilvl 876 BM Hunter.

1. 
   
2. Strip down. Less gear, less damage you'll be dealing. Be certain to have a few healing potions or a class with some sort of healing capabilities, just in case.
3. 
   
4. Start the encounter.
5. 
   
6. Hardest part: do nothing. Don't attack the dwarves, do not use the harpoons. Allow the dwarves to kill off your engineers. You're waiting for Razorscale to enrage. You'll see the following notification in your chat log when she does: Razorscale goes into a beserker rage!
7. 
   
8. Once the engineers are dead and Razorscale enrages, trigger the harpoons.
9. 
   
10. As soon as Razorscale lands, punch her a few times and bring her health below 50% to keep her grounded indefinitely.
11. 
    
12. From this point, simply stand right in front of her and allow her to cast [spell=64021] repeatedly until 25 dwarves have died to it. Takes 2-3 breaths to kill off the dwarves.
13. 
    

I highly recommend doing this encounter on a Hunter for Feign Death to reset the encounter if anything goes awry or if you did not collect enough dwarves.

Good luck!

Here's how you do this, I just did it on my 120 demon hunter (ilvl 433).

1. 
   
2. Take everything off first of all. Even if it's not necessary it makes all the difference in case you hit razorscale or a dwarf and accidentally kill it.
3. 
   
4. Next, start the encounter.
5. 
   
6. Do nothing, watch the dwarf engineers make the harpoons, get wailed on by some mobs, but do nothing. Don't fight back, don't help, don't attack. Nothing.
7. 
   
8. The harpoons will become ready, and then engineers pull back. Let the dwarves kill the engineers
9. 
   
10. Once engineers are killed, Razorscale goes berserk and there's a raid message saying so. IMMEDIATELY USE ALL 4 HARPOONS.
11. 
    
12. Bring Razorscale down to the ground but do not attack. Kite the dwarves to the front of her mouth. Wait.
13. 
    
14. Let razorscale do her blast thing, and you get credit for those dwarves killed. She breaks the chains and flies up.
15. 
    
16. Fall off the edge of the cliff and release spirit.
17. 
    
18. Teleport back to the colossal forge, restart from step 2. Repeat until 25 dwarves are killed, you will see the tracker in your achievements menu
19. 
    
20. Once 25 dwarves are killed, punch Razorscale. 2 or 3 is enough
21. 
    
22. Get the achievement
23. 
    

Completed as of Patch 10.1.5.

In order to complete this, you will need Razorscale to use her frontal attack while grounded in order to kill the dwarves. However, if you bring her down using the harpoons, she will do a single breath and then return to the sky and render the harpoons unusable going forward. Here is how I completed this in 10.1.5 where taking your armor off and punching her to below 50% no longer works due to the scaling of how much damage your fist do:

1. Talk to NPC to start the encounter.
2. Protect the Engineers while they repair all 4 harpoons. Do not use the harpoons.
3. From here, I waited until a good chunk of dwarves spawned. (I used the addon Instance Achievement Tracker to tell me when 25 dwarves were spawned.
4. I then grouped the dwarves while I clicked the harpoon. (Make sure she has enraged as others have noted before bringing her down). She will be stunned for a short time and then she will do her frontal attack, killing dwarves.

Now, at this point, you would get her below 50% and she would permanently stay grounded. That isn't possible anymore. So, instead, you can run to the ledge on the other side of the arena, jump off, and then restart the process until you reach the total of 25. (It will keep your progress from a previous attempt).

I ended up doing 23 in my 1st try, then finished it off in the 2nd try and killed the boss after getting the achievement before she flew back up.

Hope this helps!