Same concept as the single player wins. Start an online 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 not too long to wrap up the achievement from first win through the 50th.
Yes, this is mostly copy paste from my single player wins guide.