This is a rare drop in the Raid Mode, you have to be lucky.
Every time a puffer fish shows up, kill it! This will give you a weapon crate which can contain a super rare weapon. These fishes show up AFTER killing all enemies in the area.
In the store you can buy Rare Finder which increase your chance of finding rare weapons. These cost up to 200000BP.
This is what the fish look like:
Places to find puffer fish are:
- Stage 2 / at the very left of the beach at the shore
https://1drv.ms/i/s!AmGvUw2bxUKgmnnu8nimu5dGFnk2 - Stage 4 / after jumping down the second time, room with crates in the middle
https://1drv.ms/i/s!AmGvUw2bxUKgmnoV0RR4hpcrdr4o - Stage 5 / before you finish the level in the last bathroom
https://1drv.ms/i/s!AmGvUw2bxUKgmnsymJ2ZHiCMnlQB - Stage 10 / right at the end
https://1drv.ms/i/s!AmGvUw2bxUKgmnnu8nimu5dGFnk2