I'm creating a simple 2D Shoot Them Up, all graphical aspect is made. Now I'm thinking of the enemies AI. Do you have some ideas on the way I could make them have believable actions and reactions. cheers

The AI of shmups, is not believable or reactions, but the exact opposite. It's predicability and patterns.

Humans, expecially humans that enjoy shmups, enjoy finding patterns in how your enemies come at the player and finding paths in the spray of enemy bullets. The perfect game has one or two rail-shooter-esque path that the player must find to sucessfully navagate the level and avoid all those little pink bullets.

The concept of a wave is pretty standard, where several of the same enemy attacks you in a line, or they come at you one at a time, one from one side of the screen and one from another. This doesn't mean you can't add some randomness to throw a player off. You can have different waves attack at different times, but trying to be unpredictable / realistic is usually not prefered in the genre.