Now not all of them seem to work right on my phone (It's a Samsung Galaxy Centura) but one that does is MikuMikuShot. What you do, is you tap the bottom of the screen non stop. This makes Miku shoot out Negi. You can't stop, not even once, because the enemies will come flying at you lightning fast.
Some of the enemies are Anomaro models. They're nice, but I can't get into non-editable models. The enemies I found so far are Luka Megurine, Black Rock Shooter and Gumi. I think Gakupo is in here too, but Luka has a bad habit of sneaking up behind me, so I can't seem to get that far.
If you need a free stress reliever, this is definitely your app.