After many months of battling MC's bugs and terrible odds, I have finally succeeded in assembling the challenge room thus getting a shulker, an elder guardian, a wither skeleton, and a dolphin in the same room.
Many thanks to those who helped me and those who contributed !
Here is a showcase of the final build and after that, some pics about the adventure that bringing those creatures together was !
Functional machine - [will update interior soon]
Pics about the adventure of getting the shulker out:
Failed attempt #3 :
Out of the end - one hell of a challenge extracting it from spawn :
Daily voyage for around a month in the overworld :
Pics about getting the elder back (different machine design)
Thank you again to all those who helped !
Special thanks to @Eberk for the guardian's blessings