You can access the secret aegis lab zone from the mine caves (South east of the volcano).

I have found a solution to my problem: You need to go build mode and free cam (AKA god mod) take a sleeping bag and drag it to the lava zombie boss. here is a video to help you do it (https://www.youtube.com/watch?v=eXnbbwMJ4yY) you can only do this if you are in single player or the admin of the server. I recommend also to spawn one hell's fury with one extra drum and should be able to kill the zombie in seconds.