I understand where your coming from, but their not going to turn Gantz into a mech manga. I bet that mech that the seven time winner is piloting took ages to get. (Most likely multiple wins which NOBODY from the Tokyo team has) But of course I know Nishi is more badass than what we have seen of him. But IDK how he can survive after what just happened to him.
As for the Oni Alein, you cant compare that alein to this one.Nurarihyon is worth 100 pts alone, not inlcluding the fox demon(68) and the other demon alein(71). There is more than 200pts worth of aleins. The Oni mission was hard, but nothing compared to a shape-shifting, regenerating alein.