here is a though
lv 0 would be the weapons that are int he room, short gun, y gun, rifle, sword, bike and suit.
lv 1: we don't know izumi was supped to get one for getting 100.
lv 2: no idea but that would be something that you get for clearing 2 times.
lv 3: well i am assuming those are the pancake guns that those osakas had for clearing 3 times.
here is another thought.
i a making the vague assumption here that every time you clear and get 100 you get a new and stronger weapon but when you clear it more than once it could be that gantz takes the ones given last time and replaces it with the new one so as to no have other gantzers use it like nishi did.
We saw how strong those pancake guns are i mean i would just go around killing 1 point bitches till i get 100 lol 2 blows and you get 100 ezzey.... besides that we see that those guns don't work on 50pointers and above. the reason i am saying that is because we saw how those boss assistants survived it. the same goes for the y gun and the regular ones, although they can deal damage its not enough to kill them. the sword is in a category of its ow because its the only non gun weapon, that and the the suit which are both vital it seems. Some weapons are more affective vs some aliens and others not, we saw that in my theory the sword killed those two mini bosses while the pancake guns couldn't, the reason for that is because the sword is mean to cut only, seems like its infecting certain types of aliens like nuri (but i could be that nothing below a certain level of weapons can affect it, i would take a wild guess but lv 5 would be a minimum). from the little story katou's new bitch said oku beat the last 100pntr alone which means that he has the tools to do it, which brings me to my point you need a certain level weapon to be on par and even beyond that of a 100pntr.
Well feel free to bash this but it make sense in my head.