At 70% and 40% all the bombs in the whole room will be triggered by the first boss. That means that you shouldn't touch 15 or more bombs by that point. What makes the situation harder is that this boss casts sabotage every 10 or 15 secs on a random player. In order to avoid triggering a huge number of bombs, you need to run towards a corner, so the sabotage will burst into two directions. Don't forget to count the bombs, and make sure you run into one of the corners. When you see that 15+ bombs are left, you can dps the boss until his HP is 70/40% — and that's all! The achievement is yours!
Once you've finished the second phase, you should return to the point you'd came from. Keep moving until you see the house (you also may call it a pillar), then there'll be a huge hole in the wall. Get into this hole and you'll come up to the last boss. After you kill every enemy, your leader of the raid may increase your damage by 100%.
In the 3 phase you have to defeat a few bosses located inside of some of the Pandaria instances. Kill the mini-boss and you'll be able to see that a little potion at his desk starts glowing. Take the potion and you'll be transformed.