The climax of the final battle. Achilles defeats Alexander by climbing into the hand Athena sliced off earlier in the fight and firing Alexander's own Rocket Punch into his robot, making it explode! It stretches the Willing Suspension of Disbelief since Achilles took the same blow to the chest in their first fight and his robot was just thrown off-balance, but Rule of Cool demands it be allowed anyway, consistency be damned.
The book, derived from an expanded version of the script, explains that the fist is supposed to explode, but a defect caused it to not detonate. The technicians fixed it for the second fight, and the fired fist next to Alexander's robot as it explodes is a filming error.