How does Master Yi's ultimate interact with Viktor's Gravity Field?
Solution 1:
Because Master Yi cannot be slowed, he does not receive the debuff or any stacks, preventing him from being stunned.
http://na.leagueoflegends.com/board/showthread.php?t=2052671
Solution 2:
Master Yi will have the "cannot be slowed" effect constantly popping up over him. You can try it out in game.