Uuhhh that's the worst idea I have heard in this forum in the last 12 months. Easily.
What could possibly make you downgrade to an unsupported version?
Yes that's true, the premium support would end soon however, there are certain reasons for which we want it to be downgraded.
Can you please suggest the best approach to execute this task?
Take a workstation with 2 screens, open the 12c OBIEE on one screen, open the 11g OBIEE in the second screen.
Start recreating every single object by hand, one by one.
5 dashboard, 40 analysis (a report is a BI Publisher report, I doubt you have those) and few more things will not be too long.
This will give you the cleaner downgrade possible.
(and I'm not even joking on this! )
Of course you could try by copying the XML around between the versions, but you could have unsupported XML not allowed by the 11g XSD or things which are allowed but not working in that way.
So any automated approach could take you long to compare and debug. Being such a small environment in 1-2 days you must be done by recreating things (and why not try to implement some minor cosmetic changes you have maybe been asked for? Would give you an excuse for justifying doing it by hand if somebody ask )
I'm totally with you on the fact that a move back to 11g isn't a solution to a failed move to 12c. He said he has "certain reasons", not giving a single one. This sounds like somebody somewhere decided that, and we can't do much against "smart people" knowing better.
Of course they better try to solve the "certain reasons" and stay on the only really supported and evolving version (12c), but the OP sadly isn't asking for that.
And trust me: I hate myself enough for having posted what I posted above...
And most likely the OP will never ever look at his thread again and just blindly do this without thinking.
Gotta love IT...