Easiest way to get this would either be in:
1- the Fleet Battle Training (needs confirming - looking unlikely)
2- a solo fleet battle on Easy (how I unlocked)
Both of these are just by playing the match as you normally would, and using a Y-wing or Bomber to take out the key targets on the Capital ships
To ensure you get the final blow, I'd have a bomber with a good stock of bombs (either respawn, or resupply at a frigate)
Then wait until the flagship is nearly dead (using the bottom semi circle of the target.
Then aim for an undestroyed system or section of hull (so not on fire) then let loose with all yours bombs
To start a Solo Vs AI Fleet battle:
-head to multiplayer and training
-select Fleet battles Vs AI (third one down)
-should automatically be on Solo, and then change difficulty to easy