Who came against King Asa of Judah with an army of a million men and 300 chariots?

Answer: Zerah the Cushite

Asa cried to the LORD for help against Zerah's vast army at Mareshah, and the LORD struck down the Cushites, who fled.

“Then Zerah the Cushite came against them with an army of 1,000,000 men and 300 chariots, and they advanced as far as Mareshah.”— 2 Chronicles 14:9 (BSB)