The most difficult Shoot 'em up game for iOS is Fornax.

Fornax is a Shoot'em up game with RPG elements for iOS, in which you control a ship with 12 parameters and 85 possible improvements.

An unforgettable atmosphere.

Fully generated levels. 

Boss fights.

Bonus and secret levels.

Test mode after completion of the main scenario.

Three levels of difficulty.

Adaptive gameplay.

15 studies.

The possibility of repeated reinvention with the preservation of research and extracted crystals.

No ads.

Fornax gameplay video 

AppStore:  https://itunes.apple.com/ru/app/fornax-space-scroll-shooter/id587382602?l=ru&ls=1&mt=8