Level 172:
First, make sure you have a gun and gc if you can try to use an arsenal.
Second, just lure the npc's to the edge of the map and do a 3hit combo and gc them. If you have and arsenal he should kill anything else
Third, On the fourth wave there will be a mini boss. Just shoot him over and over. There will also be a ganda you can shoot him if you want.
Fourth, Fifth wave same thing.
Lastly, At the end do not go to the boss. Make sure you have at least 2 bullets saved up. Move to the middle of the bridge and shoot foward.
Then you should be done. This is how I sectored from level 1 on my second account