Usually 6x6 refers to 6 allowed elements and each element having 6 allowed cards. This adds to up a list of 36 cards being allowed in the tournament.
Normally the TO picks the 36 cards before the tournament. 6d6 makes no sense, though. There are 12 elements.
I'm assuming it is 6d12 first to select the 6 elements, and reroll a d12 if we get a double.
From there you can roll for different cards within each element, though that takes a lot of time. Id rather just have the TO handpick the 6 least used cards within that element.
Certain cards are too strong to allow in such a format. Rolling multiple nymphs is not something you want either.
Suppose that flying weapon, green nymph and druidic staff are allowed, but adrenaline isn't. In that case people with the nymphs have a great advantage, while it normally isnt a big deal (with adrenaline allowed).
Suggestion: shall I make a 6x6 slowbuilding for next week and pick 36 less used cards? Its a really fun format.