I used a pretty lame strategy to get this achievement. Basicly, if he is transformed into the rhino form, and you drop aggro, while he is running back he wont switch back to human form, and he is pullable. If manage to pull him then, and kill him, you got the achievement.
What we did was have 1 prot pally heal me (we had 2 tanks for the share the love achievement), and after he transforms, the pally DI's me. I use my macro to remove the aura, and quickly heroic throw. The rest of the party who was waiting around the corner runs in, combat ress the pally, and finish it off.