i was going to comment the same thing. i still haven't gotten this one

1. Pay a random Shaman to spec into Reverberation.
2. Have shaman do nothing but spam Wind Shear. Also, bring a rogue and a prot warrior to nothing but spam their interrupts too.
3. ????
4. Profit.

Have DPS slow down (the super fast casts only happen when hes less then 50%). Have a MAGE counterspell the last cast before the 50% mark (7 seconds to kill him after that). Burn the 50% in those 7 seconds and you win.

You get 10 seconds - Counterspell locks him out for 8 (not 7), and he only ever tries to transform every 5 seconds.

Less-Rabi is one of those achievements you can get lucky on - did a random on my hunter about a month ago alongside a well geared rogue and warrior tank. No interrupts required, we just burnt him down so fast he didnt get a chance to finish the first one he started casting.

i done it just a week ago - me as prot pala, resto shaman who changed spec to melee dps, 2xhunter and a mage. I was first to interrupt boss with my stun and all other on second casting. They didnt get a chance :) One my interrupt short before his 50% and he died before second casting :) Those 3 normal dps had (with using heroism ofc) 8-11k dps on boss.

I've actually done that achievement without him casting transform even ONCE. we were in ulduar gear (5 dps, 2 dks for army) so i assume it will be a cake now with icc gear. just take a reg group and atleast 1 dk and a tank who has an offspec dps just for the boss fight.