While adding comments on the project to sell its template, I finally realised that the Double bullet Power Up didn't have the bullets deal 2 damage each instead of 1 as all the other bullets.
After have just tried alone, I can confirm that the game can be completed also using only that weapon an only one rooster. Sorry you had hard time playing with the under-powered weapon.
I will update the game once the Game Jam will be completed, so if you want you can give it a go again to see the difference.