In the time between the first and last mission, almost a year has passed, there have been only 8 missions (counting the Shorty and Tae missions). Nishi has jus been in Gantz for a year, so since he was the youngest of his group, its possible that he took a longer time to get 90 points. Also, if the rest if the team were like Izumi, then the group work that helped the current gantzers (Reika, Old Man, Sakata, etc.) didn't exist, making it hard to get more points (remember that the current characters trained between missions). I think that Nishi started like Inaba and eventually got used to the missions and on killing the spoils of the other gantzers. Also, he depended on the newbies because they served as cannon fodder.
On top of that, I think that the lack of help was vital in his death (the ruined suit doesn't count since Kei has been able to survive without it)
After all, Nishi is the one who knows more about the missions (he is from Izumi's time, so he has been for quite a while in Gantz), maybe not too much about the weapons, but at least more than the current team