For the second attack on Ai, what did God tell Joshua to set up behind the city?

Answer: An ambush

God told Joshua to set an ambush behind Ai, and this time Israel could carry off the plunder and livestock.

“And you shall do to Ai and its king as you did to Jericho and its king, except that you may carry off their plunder and livestock for yourselves. Set up an ambush behind the city.””— Joshua 8:2 (BSB)