Prago, as a K2-Phantom had many opportunities to become stronger, much more so than Adam or Edir, who were only K1-Phantoms. However, Prago did almost nothing to accomplish this.
Someone like Tron was significantly stronger than him, not only because of his toughness, extensive combat experience, and formidable aura but also because Prago had barely reached his potential as a K2-Phantom.
In order for Phantoms to become stronger they had to constantly visit Dead Lands, fighting and risking their lives there. It was a path to strength - proven, effective, but dangerous.
Sure, Prago had been to Dead Lands more than once, otherwise, he wouldn't have been able to become K2-Phantom, but he always chose the safest and most reliable missions to develop slowly but surely.
