Here is a better tutorial that I have found, has to be done on normal.
So the fastest way to get this I found is with 2 people and have the Elixer Talkin' Bout Regeneration or you super for extra health.
Old written solution for those interested:
Only one person gets the achievement at the moment.
If multiple people reach the beach it favors the host.
You only get one launch per game to the secret place.
But you can get launched there with up to four.
Has to be done on normal.
So the fastest way to get this I found is with 2 people and have the Elixer Talkin' Bout Regeneration.
I'm gonna assume you know how to build the flinger up in the facility, get dynamite and build the shield.
Just both run in a square with a normal zombie on the flinger for 15 seconds and then just stop on the Left side. (The side facing the zipline you came from). Badabim badaboom you get launched to the place.
Pop the elixer when you land in the water or start to take damage during the swim. Either way you will make it to the beach.
If the wrong person gets launched for whatever reason. Just restart the game.
There is only one achievement per game at the moment.
A little more detail.
*** Spoiler - click to reveal ***
If you just open what you need you will be done at round 7. Going more rounds shouldn't matter. I just like to get done quick.
Work on the dynamite ASAP get 1 fire zombie every round from round 3 onward so you're done at round 5.
You can build the shield before you go up to the facility.
6 is a dog round.
So open up the map to facility power to get the gearbox from the flinger.
Have someone save a normal zombie at the facility while someone fixes the gearbox at the hermit in the lighthouse.
Place the gearbox again.
Just both run in a square with a normal zombie on the flinger (stay inside the yellow for refrence)
for 30 seconds and then just stop on the Left side. (The side facing the zipline you came from).
Pop the elixer when you land in the water or start to take damage during the swim. Either way you will make it to the beach.