Iron Dwarf, Medium Rare (10 player)

I was trying to achieve Glory of the Ulduar Raider (10 player) and I also saw that the counter of Iron Dwarf, Medium Rare (10 player) was not working really well.

After some tests, I finally understand how to be sure it works.

First, it's obvious that only Dark Rune Guardian will count in the achievement.
Both breathes from P1 and P2 can be used to kill guardians.

The thing you need to do to make it works is DEALING DAMAGE (at least one) TO THE MOBS WHEN THE BOSS IS ON THE FLOOR !

So, aggro mobs when the boss is flying, but keep in mind that you need to damage them a bit after using the turrets, without killing them. After doing this damage, every mob killed by the drake will count for the achievement.

After 6 or so years, I've finally made an account to post this comment.

I've found the guaranteed way for each Guardian kill to count towards the achievement, and just got it today. The reason that the kill counter seems so buggy and unreliable is because the Dark Rune Guardians must take damage from you (or the group, if you're not soloing) AFTER Razorscale lands.

Being 6.0.2 with the extreme damage bonuses we get now, the difficulty in the achievement is damaging the Guardians just enough so that her fire breath will kill them. Post 7.0 Edit: And now, finally, after years of achievement strife, Blizz has finally delivered unto us this Soft Foam Sword, which will make accomplishing this achievement less of a goat rodeo. You lucky dogs...back in my day, we had to strip down naked and punch the mobs to death.

More shameless editing: The Soft Foam Sword is a big fat phony, just like my first Wowhead post.

Right now, each fire breath hits the mobs for 7,500 damage, so shoot for just below that number. (Once you hit the 15 minute enrage, it's a moot point, and the whole thing becomes much easier.)

Recommendation: Hit them once to gather them up, then bide your time until you can pull Razorscale down again. Once landed, melee each Guardian carefully until you've got them below that 7,500 mark. I guarantee each kill will count.

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Edit: Jules93 far above me realized and stated this long before I did - I just overlooked his comment. My first post on Wowhead is a phony! A big...fat...PHONY! D:

Just did it as a ilvl 880 BM hunter.
It is easy and automatic now, don't listen to all that fluff people are saying.
DO NOT take off gear
DO NOT shoot Razorscale
DO NOT time your AOE
DO NOT sweat

1. Start the encounter, protect the engineers until they build turrets
2. One turrets are built let them die
3. Once all the friendly troops die the mob comes after you
4. The guardians will keep spawning in the back, once you have enough go collect them, feel free to kill Watchers
5. Once you have at least 25 Guardians banging at you go back to turrets and shoot occasional Sentinel that may run towards you
6. Wait 10-12 minutes until Razorscale enrages (it'll have buff on it saying damage increased 900%)
7. Shoot turrets, Razorscale lands
8. Move directly in from of it, in less then a minute Razorscale breathes and everything but you dies.

UPDATE: Seems like in 7.3.5 due to scaling issues Razoscale no longer one-shots the mobs. So this means as step 9, once she lands you'd need to GENTLY (meaning you might want to unequip or sport a fishing rod as a weapon) bring her health to below 40% to make her stay permanently on the ground and sit through few iterations of her breath until she kills the mobs.

UPDATE: IGNORE THIS - people say toys can no longer be used in instances: [item=137663] is a perfect weapon to use in this case.