Kill an Abadon Mutant Crusher with the Phoenix.

First you will need to find an Abadon Mutant Crusher that is not in the sewers and these are few and far between. You can make your way to 'D and D's Hood' in The Wilds area of the map and on the island before where they are, you can call in your Phoenix and then make your way, precariously, across the makeshift bridge into their lair. Once here, be aware that there are 2 of them and they will destroy your Phoenix with ease, so you will want to only attract the attention of 1 of them and try to bait them away from the other, then use your weapons on the Phoenix to bring it down. This can be tricky however given the small space to work in.

Alternatively you can fast travel to the Mutant Bash TV location in the Broken Tract area of the map and head out of the front gate and then turn to go North-East, an Abadon Mutant Crusher has a chance to spawn here, making it a much more viable place to unlock this achievement. If the Abadon Mutant Crusher does not spawn, you can fast travel somewhere else, then back to Mutant Bash TV and run here again to see if it spawns and repeat until it does.