Try it with 5 DPS.

I got that one with all dps and a pally in good ret gear as a "tank"

Bring a smart shaman, it does wonders.

I tried and tried and tried doing this the "lets line up enough interrupts" way and one would always miss the cast timer by a hair. Brute force works better.. He can't transform if he's already dead. It's actually dirt simple with a tank and 4 overgeared DPS (one should, of course, be a shaman for heroism/bloodlust). You should only need a single interrupt (maybe two) with enough damage output.

It's easy with 5 dps with decent(over 5k) dps and a shammy for heroism/bloodlust.
Just ask the tank and healer to go dps spec.
Basically you dps like mad, interrupt his first 3.8 second cast as late as possible, and from there you have 5 seconds to kill him.
Usually ends up in a 12~13 second kill with an average dps of 6.6k per person for that 12 second duration. Most pure dps classes can burst much higher with cooldowns and heroism/bloodlust during those few seconds even if their average dps is lower.

Twice I had almost accomplished it because the random group i was in had downed him so fast that he only tried to transform twice. But when I finally got it, it was just from me interrupting him constantly on my BE DK. On that attempt he tried to transform 5-6 times but I managed to interrupt them all.

I got lucky and managed to get this one with a random dungeon group and no one ever mentioned anything about doing it.

If you get a group together that can kill him in 15 seconds or less, you generally only need one interrupt. I've done it dozens of times now for guildies, some with 5 DPS, and then others with 4 DPS and 1 healer. The key is to not interrupt the first transformation until it is around 75% complete. The cooldown on the ability resets only after the interrupt. That gives you a few fractions of a seconds more to down him before the next transformation.