The Slam contract is rock solid, provided you play it correctly : losing a trick early !!!
You have 2 sure winners in ♠s + 2 sure winners in ♥s + 6 sure winners in ♦s, a total of 10 sure winners.
Holding K♣ Q♣ J♣ the remaining 2 winners should of course come from ♣s.

Therefore win the first trick with the K♥, then immediately lead a small ♣ to East's K♣.
If Opponents don't win this trick with the A♣, play ♣ once more leading the J♣ (High cards from the short side first !) from East's hand.
After one Opponent wins the trick with his A♣ he will lead another ♥ : win the trick in West with A♥, then cash East's A♠, K♠, A♦ and K♦.
Now lead 10♦ and take it over with West's Q♦ and run the remaining diamond winners and finally the Q♣, making 12 tricks in total.