Cloudberry Kingdom

After spending awhile trying to get this by myself I found a really easy solution.

Firstly you must have 4 controllers, i ended up using my 2 regular controllers and my 2 arcade sticks.

Go to free play mode with these settings:
Game: Bungee
Hero: Spaceship
Difficulty: Unpleasant
Length: Really your choice but I found making it the shortest worked well

Use the controller getting the achievement in your hand, have the others on the ground. The ground controllers use rubber bands, or your toes to hold down on the left analog stick, or position the controller so that it is forcing the analog down. The point is to kill the other players asap.
The more friends you have helping you the easier it will be obviously.

Simply touch the door when you are the last alive and it auto closes. "Dead" spaceships do not weigh you down or affect your flying. I thought this was the easiest way because no jumps are involved which with 3 dead teammates weighs you down usually preventing you from being able to make jumps correctly. Should take no more than 2 tries.

Bungee levels are an option in free play mode. Essentially, it's a bungee cord between all players. Since the achievement says three allies must be dead, you have to have four people (or controllers) in the level.

You can do this with three other people, but you can also do this by yourself if you have four controllers. If you decide to the the former, just take turns killing yourselves and letting one person finish. If you decide on the latter, make sure the controller getting the achievement is either the first or last one, turn up the aggressive settings, and keep making random levels until you get something that will kill you at the beginning. From there, make the other three controllers (starting with the one on the opposite end of the cord from the achievement-winning controller) kill themselves with it. Once your three "allies" are dead, complete the level and the achievement is yours.