Nishi was far less than impressive in all of the missions we got to see him in before he died. The aliens in the begining were insanely weak compared to the ones now.
When fighting against those bird aliens(can't remember the names off the top of my head) and the onion alien, he relied on his invisibility completely. Even when he was fighting, he didn't seem to consider even locking on to the alien with his gun either, which would have saved him a lot of trouble.
My theory is that in the old Gantz group when he was with Izumi, they were all loners who never worked as a team. And Nishi was probably the weakest of them all since it took him so long to even get up to 90 points while (assumingly) all the other, stronger characters were already released after their 100 points leaving Nishi behind. The old Gantz group could have made him learn everything by himself, without any help. Which would explain not knowing to lock on with the guns and never bringing the sword with him.
Even now, when he's brought back. Whenever he is confronted with a tough fight with the vampire guys, he hides behind his excuse of being in his "Off duty time".
I'm just saying that Nishi has been all talk and no action in the whole series.