BLUF: replaying the Inferno operation until it pops. You must complete the level "Skyfire" in the campaign to unlock this operation. This should pop during your second or third run.
Recommend going with bots (maybe cancel matchmaking when you're in the rectangle that'll trigger the mission from the battle barge) as they're usually much better at letting you deal final blows than randoms. You can compound your efforts for this with the class-specific achievements too, which will all take MUCH longer than this anyways. DO NOT play offline to guarantee bots because progress won't save.
If you're too eager to serve the Emperor's will and bite a group of enemies that you can't chew, then you'll see that the bots are really good at reviving you and holding their own. Practice parrying: you don't need a blue light to parry and you can parry every melee that's not a red light to help with your armor, killing bigger enemies and stunning smaller ones.