It is one of the freemium ones. You have ads running at the top of the screen, even the game area. You choose one of the monsters on offer and battle against it. Each monster has minions that you fight first. You collect diamonds that you can then use to buy upgrades (weapons, extra lives, etc).
New bosses get unlocked as you defeat the previous boss. You can also choose to fight a stronger version of a previously defeated boss for greater reward. You also have the option to buy diamonds if you don't want to grind.
What can I say... If this game was made by some 13 yo schoolboy, I'd be very impressed. This was developed by Backflip, however. I really respect this studio and expect more from them. It looks like with their latest string of games they are becoming more like Glu. Release a free game made in haste with little innovation and style and hope that it would do well. Perhaps economically that would make sence, but I am really disappointed. I felt bored after I defeated 3rd boss and took the game off my device. I hope the studio doesn't go down the same path as glue and would release games of the same quality as Ragdoll blaster 2 and Army of Darkness.