I can literally answer this question! Hardest: Samira.
Champion double kills during ult - lowest increment rate in the game (fiddlestick's It won't die is close 2nd)
Champions "juggled" with passive knock up extension - she can't actually do this one herself, so she needs allies with knockups to have a chance.
Ultimate projectiles blocked with W - if you're against ashe or jinx, you'll get 1-2 per game. If you're against Lucian or MF, it's more like 40-50 per game.