Shouldn’t Cross be one of the strongest people by now? I feel like he keeps getting nerfed
Not really. He is certainly strong but not among the strongest people because the author keeps making a deliberate choice to avoid a direct comparison.
For example in the last fight while he struggled to beat one of the jamming beasts, it was clearly mentioned how the other two people beat all the monsters in their path plus beat the jamming monsters way earlier than Cross, without even incurring any injury, while one of them was even protecting the princess. Clearly establishing that he is below them.
Heck even since the Gimlet fight, we has been given hints that part of the reason why Cross won was because Gimlet underestimated him and let his guard down and also that he didn't realise the sheer breadth of Cross' abilities.