Going back to tsuwm's original query:

[here are two that I've been pondering:

1) "it's the absolute opposite of a no-brainer" - given that a no-brainer is defined (by M-W) as something that requires a minimum of thought, what would the exact opposite word be?]

Would this be a "full-brainer"? - meaning that the question/problem is difficult and therefore needs the full power of whatever brain you have?

edit
For instance, if you are acknowledging that you have limited experience in solving a particular problem, you might say, "!You might find this easy, but for me, it is a full-brainer."

Last edited by Rhubarb Commando; 08/21/2012 9:25 AM. Reason: additional thought
I'm immortal until proven otherwise