Beat the game...
Not going to go for any special achievements. The gameplay was overall just too repetitive. It was unnecessarily hard in the beginning, but once you get access to explosive and freeze abilities and at least one sling shot upgrade, the balance shifted to the point where I took practically no damage from waves 12 to 19 altogether. Wave 20 was also trial and error. Though I beat it at my first time there, you can't really expect the player to know what to do on a whim...
I suppose it's always difficult to find the proper balance in these things. The order of the food unlocks could also be improved. I'd suggest having a spread ability first and then a pierce through, because most of the difficulty relied on the fact only one enemy could be hit at a time.
It's a fun pass time, but there isn't much to separate it from the crowd.