This achievement is currently bugged in our favor. You only have to save the first two angels that you come to after you speak to Abadon.
You will come to a small ledge where you can see two white angels below. These are the guys you want to save. When you drop down the purple angels will spawn. One on the left side of the water and 2 on the right side. As soon as you dispatch these. 4 more purple angels will spawn. Two on the left and two on the right. This is very difficult to do solo and I highly suggest waiting to do this after your characters are fully powered up and you do it on casual difficulty.
Once you have killed the 7 purple angels without losing the 3 white angels then the achievement will pop. The white angels die REALLY fast if you don't take out the opposition. They are also stupid and will stand in AoE from the purple angels and die as well.
If done successfully abadon will come out and thank you for saving his angels and the achievement will pop.