You have to use the hard style and block 250 attacks with your shield. You can do this in Arcade mode with a second controller. Before you do this go to the settings and set the time to infinity and the rounds to 5.
Use any Character you want (I used Kurisu for both players) and put your main account into the left corner and the second account beside her as close as possible. Press forward with your main controller and
for a kick with the second controller at the same time. If your shield turns blue you have the correct timing and should continue pressing these buttons. You can collect a huge amount of protections in a single round with the right timing.
If you still have questions feel free to leave a comment! =)