This milestone is awarded for shooting down 80 starships.
Space pirates are the obvious targets for this - you'll encounter them at random throughout the game, either in small groups attacking you (more often when you carry valuable cargo), or in larger groups attacking freighters.
There are also sentinel ships that will appear if you misbehave (eg. shoot at a station/freighter, or reach wanted level 4 on the ground).
Combat tips:
- Hold L2 (brake/reverse) to turn faster, to track ships as they fly past.
- Keep a supply of oxides on hand so you can charge your shields if they go down (just open the menu and feed it as you normally would, only faster). Titanium is ideal for this, it's more effective than iron but just as plentiful since you can farm it from sentinels anywhere. Good news: there's a quick refuelling menu now (down on the d-pad) so you don't have to scrabble through your inventory to keep that shield up. Bad news: sentinels don't drop titanium any more.
- You don't need two weapons! Pick one, uninstall the other one, free up all those precious upgrade slots.