Got "less rabi" on the first real try (previous try I fat-fingered it) thanks to some of the advice here and on the wowhead forums. Basically, it came down to Improved Counterspell, which I just happened to have available to me since I recently swapped my mage main in for a shaman.
I started off kinda slow so that I'd get the first interrupt right before 50%. Interrupted that with wind shear and then proceeded to the 50% interrupt, also with windshear. At this point I just waited for the 5s until the next cast. IOW, I did very little damage to him (he might have been at 45%) so that the cast would be easy to interrupt. At about 4 seconds (1 sec before cast), I blew all my cooldowns, interrupted the cast with Imp. CS, and killed him before he could cast again (you have about 10s).
It would have been much harder without Imp CS, so I guess the best idea would be to try to time the interrupt. <20% health casts are almost instant, don't think you can react with internet lag.
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