As far as free-to-play titles go, "Boss Battles" is surprisingly compelling thanks to its thoughtfully designed baddies and thorough upgrade system. Definitely worth a look if you can handle in-game ads and don't need bullet hell challenge to enjoy a scrolling shoot 'em up.
Defeating an alien invasion from your iPhone has never been this much fun! Gamers in search of a hardcore challenge will especially be interested in checking out this sometimes vertical, sometimes sidescrolling, and always intense shoot-em-up.