Saleen's Elemental Eye advanced to become the Eye of Souls before he realized it, enabling him to see more and giving him a big boost in battle. Since that was a passive ability that evolved on its own, consumption would have been over 100 times less than what usual Eye of Souls would have entailed.
When common mages used the Eye of Souls, they did their best to shorten its usage to prevent them from exhausting the power of the soul.
It was more difficult for power of the soul to recover compared to mental powers. Having the former exhausted in battle was extremely dangerous.
While Saleen was busy destroying Cologne City, Rafel was doing a great job killing the Heaven Knights. Since the Floating City had already destroyed the elite parts of the Heaven Knights and caused half to die or be injured, the Bug Angel Warriors and metal angels led by Rafel made short work of what was left with them.