The venerable hack-and-slash is a game genre that transfers well to mobile devices, and there are plenty of examples on Android. However, many of them are loaded down with in-app purchases. I'm looking at you, Dungeon Hunter 5. Implosion, from the developer of the popular music rhythm game Cytus, is a cool-looking hack-and-slash that only charges you once. It's expensive, but you can try before you buy.
Implosion is set in a post-apocalyptic future as a mysterious enemy called XADA has launched an attack on humanity, hoping to drive us to extinction. Only the War-Mech series III battle suit can save us, and it looks as neat as the name makes it sound. It's a top-down isometric game with a neat combo system that introduces just enough variety to be more than a blind button masher, but not tedious to play on a touchscreen.
This game is free to try on your Android device of choice. It's a 1.1GB game with fairly heavy graphics, so you'll want to make sure your phone or tablet is up to it. The demo includes six levels, after which you can buy the full game for $9.99. Not convinced? Did I mention is has Play Games cloud saves? Swoon.