Crushing Bones and Cracking Skulls

For everyone having trouble getting hit by the quakes, here's what worked for me.

Make a macro of /kneel or /sit and put it on something you can spam (I put it on "X"). Grom uses his skullcracker twice. Once at 66%hp and once at 33%hp. I figured my damage was way too high for him to do controlled bits of damage before he reaches 66% and 33% so I unequiped my weapon.

As you enter the fight start spamming your /kneel or /sit macro. You will notice that ALL quakes hit you now providing an Angered Earth with each quake. Once you have 5 AE's up (easier to see when enemy health bars are shown, I suggest you activate that in your UI settings) hit grom with anything that gets him just below 66%. He will use his skullcracker and kill the 5 AE's.

Once he killed the AE's rinse and repeat all the above and when you have a 2nd batch of 5 AE's up hit him to just below 33%. He skullcracks again and another 5 AE's go down. Kill Grom once they are down and enjoy your new beautiful mount!

For anyone crazy out there such as myself thinking about soloing this achievement, don't bother. Quake only spawns 1 Angered Earth per player and it seems they die automatically after about a minute resulting in you having a maximum of three up for any Chains of Woe.

In short: 3 adds max times 2 Chains of Woe in the encounter = 6 adds killed max, 4 short of the achievement.

As of the 6.0.2 pre patch (and the time of me posting this) he seems to no longer summon the elementals when he casts Quake. We confirmed we were on Heroic and confirmed we were actually taking damage from the quake ability. We filed a bug report, but I wanted to post this in case anyone else was running into a similar issue.

EDIT: As of Halloween, patch notes state that it should be fixed!