Very nice first game, well done!
Basically a lot of similar stuff the others said:
- Give hint/telegraph to where enemies will spawn
- Add push back of enemies or invincibility frames after being hit to give grace period
Also:
- Add effects to game, camera shake when hit, flash characters red to indicate hurt
- Add more difficulty (do with addition of previous stuff as balancing it), lower player health
- Add more areas to explore, perhaps more where escape is so have to find