I found out that you can actually start up solo lobbies and do acheivements that do not require kills due to you being completely alone.
At main menu click "Create a Game" then choose the map you desire, so for this achievement it is "Fishtank Frenzy".
Then down the bottom of your screen, Tick the "invite Only" button and press "P2P" this will take you to a lobby and you can just press "Force Start Timer" and away you go :)
For the achievement, the Shark circles the edge of the map, so all you need to do is jump off and land on it. Here is a video. I actually just touched his back and fell off instantly and it unlocked.