After the recent update where the MAA were updated to Ghulams, the 1st wave of 10 Ghulams is almost impossible to beat, hence getting the gold is super hard now. They have increased health and cannot be one-shotted by crossbowman. Plus, the powerful charge attack combined with the increased movement speed counters crossbowmen, knights, the MAA and pretty much every unit choice.
I feel the Art of war challenges should be independent of balance updates (like the Campaign) to keep it fair and consistent. To me it seems like a bug or unintended side effect of the update.