The Astral Plane is harder than it sounds, as other Engineers can sometimes reach the teleporter first and build it before you can get there. The best option is to play as an Engineer in a game where few players are, and use the Jackhammer or Turbo Jackhammer to rush to the teleporter as quickly as possible. Try to do this where there is only one or no other Engineers for convenience. You should also watch out for any plants attempting to defend the teleporter, so it might be a good idea to use your drone and take out those plants first using either your Zombot Laser/Mean Beam or your Cone Strike (the Lone Cone isn't very effective at executing this job). It could also be a good idea to let the teleporter get destroyed, vanquish the plant that destroyed the teleporter and then rebuild it. If the zombies look like they are winning, go ahead of the garden and reach the border line of out of bounds. It is also an option to already build the teleporter anyway for a better chance to get the achievement, but the zombies may capture the base already before you succeed.