This will let you earn unlimited DP and therefore get you access to all weapons and upgrades... and all you need to do is the tutorial over and over.

So once the tutorial has ended, you'll be given some enemies to fight. Make sure you get a high combo while killing them.

When the enemies are dead, a circle will pop up in the bottom left corner of your screen. This means that the game is being saved.

As soon as the circle disappears, pause the game and go to "Restart Chapter".

Then just repeat this process over and over again for more DP.