This is the only one of the "Kill 1000" achievements that you probably will not get during the campaign. Three easy ways to get this done: 1) Replay the last Aeon mission and don't capture the control tower. Naval units will continue to attack you, just save every hour or so to see if you have enough kills yet; or 2) Keep playing sea intensive skirmish maps that have much more water than land to heighten the probability of getting a sea kill. Skirmish vs 3 - 4 normal AI's depending on your skill on Roanoke Abyss because it has a lot of water; or 3) Play with a partner in multiplayer and set up a row of torpedo turrets and have your partner send in a lot of Tech 1 units (not submarines, however). With a partner, you can do this along with Supreme Shipwright, but for efficiency you should do all of the campaigns first (especially the Aeon campaign).