Albus wasn’t sure, if Promona and Filius would be cooperative, but he knew that Minerva would help him with the chess piece challenge, because he heard from Molly that her son was good at strategy, if he was playing chess. Severus with his potion challenge would be harder to convince than Promona and Filius, but he was sure that by using the right words, the man would do everything for him. Albus was also planning to use the Mirror of Erised, in order to hide the stone inside the mirror.