Super easy to get. Start a local game against a single easy ai. Kill it almost entirely as fast as you can. Leave it with about one or two hits from your favored weapon to a single city. Whatever you can do easily in a single turn. Once at that state, save the game. Then you can win, load your save, fire your couple missiles or lasers to kill again, victory, load game, repeat. Shouldn't take more than an hour to set up, and then roughly 10 - 20 minutes to wrap up the achievement from first single player win to the 25th.