This is how I managed to get both endings and the Ranger Easy / Ranger Hardcore achievements in one playthrough. Not 100% sure you have to do all this, but this is what I did.
1. Download the DLC before playing the game or starting another playthrough.
2. You can choose Easy difficulty (doesn't have to be Ranger Easy).
3. Trigger the Enlightened ending. See edit below.
4. At the end of the game decide which ending you want. You'll either get "If it's hostile, you kill it" or "Enlightened" depending on what you decided to do.
5. After the credits, back at the title screen, choose to continue.
6. Go to options and change the difficulty to Ranger Easy, make sure you save and exit and then quit to the title screen.
7. Choose to continue again (go to options and make sure you're on Ranger Easy) and play the last part of the game again but trigger the opposite ending you did in step 4. You should now get the other ending achievement AND "Survivor" for completing the game on Ranger Easy.
8. Repeat from step 5 but choose Ranger Hardcore this time. Make sure you trigger the first ending you did again. It doesn't work if you use the same ending twice in a row.
EDIT: If it doesn't work the way I've described you may have to dashboard to get it to work as mentioned in various comments below.
EDIT: Some people haven't needed the enlightened ending and were able to use the bad ending twice in a row. Originally when I tried this, it didn't work for me that way. I had to switch endings to get the other achievement but there may have been a game update or some other variable in your playthrough that changes this.
If you have the original 1000G and just need the DLC achievements, you'll need to start a new game with the DLC installed. It means another playthrough but you'll still be able to get Ranger Easy / Hardcore achievements in the same game.