The projection of Archimonde and his true body were not on the same magnitude of strength at all.
Arthas was stunned. Only after Archimonde's true body descended did he truly understand how powerful a commander of the Burning Legion was. Even though Arthas had gradually lost many emotions and became increasingly cold, he still felt a long-lost fear when he saw Archimonde.
Archimonde spread his arms and roared excitedly at the sky. Powerful and terrifying fel flames spewed out from his palms and eyes, and the surrounding environment became cold and gloomy.