While Penny's diving lava beam was powerful, both Jack and John were pretty sure that it couldn't one-shot the supply carts. They had destroyed several supply carts during these past few days and those carts were extremely durable, even tougher than the ones they encountered during the ancient battleground.
They needed to destroy the supply carts in one swift attack before the orc army had the chance to return to guard it. Hence the need for Jack's combination explosive assault. Since the Blood Counts have killed all the soldiers around the supply carts and the remaining orc soldiers were lured away, there was no enemy to cast any spell or skill to protect the supply carts. Jack's four combined attacks should be sufficient enough to destroy the supply carts.
