I think a grand master should be the best overall with each element. Otherwise, they are just grand master of a few elements. To see who is the best overall, you'd have each master fight every other master four times. One game they use their own elements, one game they use the opposite element, and the other two they use the same element. So if it were earth vs. light, the first game would be an earth deck against a light deck, the second game they would switch (the earth master uses light and the light master uses earth), the third game they both use earth, and the fourth game they both use light. The person with the most wins (after everyone has battled everyone else four times) becomes the grand master.
That would take way too long though probably. So we'll have to settle for having grand masters that may not actually be the best overall.
Just to throw in some math, there would be 264 total games. Each master would play 44.
12 masters, so one master plays against eleven others four times equals 44
Using the "handshake" rule, you find the triangular number who's n is one less than the number of people involved
(n^2+n)/2 for triangular numbers
((n-1)^2+(n-1))/2 for our example n=12
That makes 66 "handshakes," or in this case how many games there would be if each master played every other master once
Multiply 66 by four and you get 264.
