Hi, I’m not too familiar with working with flying enemies, but I think I know something that might help, but won’t solve all the issues you may be experiencing. If you set your enemy’s movement mode to flying they will be able to move while in the air. (You can find this setting in the Character Movement Component). Setting your enemy’s movement mode to flying will allow the enemies to move in the air and stay in the constant Z axis.
Now I’m not sure how to move the enemies up or down while in the flying state. That may be something you might want to look further into yourself, but if I come across anything that might help, I’ll let you know!
Hope This Helps & Good Luck! 
