How to buy the black knight who crosses the line of fire is the most economical?

The most reliable way is to buy the hero weapon when it is on sale, but there is no guarantee that the Black Knight will definitely participate in the event and even if there is a discount, it will cost more than 600 yuan. The unreliable way is to draw a lottery, such as the May Day divination ball activity, in which there is a black knight in the prize. If you are lucky, you can smoke dozens. If you are unlucky, you can't even smoke a black knight. There is no good way for others.