A good Location to get the Collateral Damage Achievements is in Vorago Solituide. Search for a Group of enemies with a Lost Legion Ethernal, shot him below 50% HP and let him ascend. There are some versions, you need a Ethernal Magus. Now shoot the others enemies at low HP and stay close to them and let the Ethernal Magus kill them.
It’s not easy to get, but if you are Level 26+, not on TVHM Difficult Setting it should be doable, took me about 30 minutes.