Our reference game to this was cuphead so I did a lot of research at the beginning of what made cuphead such a great game. It was here we decided to make several bosses and have an ultimate and that really added to the gameplay.
We used tiled to place enemies and also the enemy's shooting pattern.
We used visual studio to playtest early and also so we could change things like enemy speed etc.
In this project, I was on the art and story task force to broaden my knowledge.