Master of One-Upmanship

X equals the highest converted mana cost among creatures opponents control.
At the beginning of your upkeep choose a creature opponent controls. Master of One-Upmanship becomes a clone of that creature and gains a +1/+1 counter. If it attacks this turn, that creature must block it if able. At the beginning of your next upkeep remove all counters from it.
"Anything you can do, I can do better"