Maybe a way to store attack sequences/enemy spawning and behavior.This thread
has a a bit of discussion related to that question.
As you use Unity, I suppose you could have each enemy be a prefab with a behaviour script attached that controls its bullet patterns. Enemy movement can be placed in the enemy's behaviour script if all instances of this enemy are supposed to move in a similar way. (e.g. "moves forward in a single direction" or "rushes towards the player")
The enemy spawning itself should be done with a level manager IMO, which is a general script that handles the sequence of events within your level.
If you are new to game programming in general and are not familiar with the concept, look up Object pooling. This concept is very useful for shmup programming as you will reuse the same bullets/player shots/enemy instances instead of instantiating dozens of them at runtime, which can hurt performance.
I think I have downloaded so much I am bored with downloading. No really I bored with downloading stuff I might consider moving to Canada or the pacific.