On the Lighthouse survival map, the best thing to do is to get a hunting rifle, then summon the hordes, as fast as you can run to the back of the lighthouse and find two rocks (one's a taller rock to the left and one's smaller and more dome shaped on the right).

Jump up on the domed one and there is a small space you have to carefully fall into.

No zombies, or special infected (except smokers, but that's why you have a hunting rifle, the smoker only can get you from the lighthouse tower) can get you.

The only other special infected that can get you is a hunter, but I have only had this happen once.

It's hard to jump back into the map, and more than likely you won't have any other survivors but yourself.

Tanks cant get you either. Have fun.