Unfortunately, this is likely to be one of the hardest achievements to obtain - entirely due to the game's declining player base.
Fleet Battles vs. AI requires four other players to queue up for the same difficulty in the same queue. After 10 minutes of searching, the queue times you out and you must start again. This achievement is disabled for offline play, and must be completed in an online Players vs. AI match rather than Solo vs. AI.
A couple of tips:
-Highest player counts tend to be around evening but before 8 PM EST
-Easy difficulty has more players than Normal difficulty setting
-GET A FRIEND TO PLAY WITH YOU. Even just one or two more players can drastically increase your ability to find a game. There are clans in-game to add that wouldn't mind helping you out.
Don't put this achievement off. This will be hard, but get it sooner rather than later. The game is less than a year old and has horrendously low player counts. Soon this may be nigh unachievable outside of a 5-stack. It may even lose server support.
Once you're in, it's cake. Go bombers and shell their frigates into submission.
Good luck!