Rise of the Ronin

In order to unlock this trophy, you need to first head to the Dojo. This is marked on your map with an icon of a little building with a curved roof:

When you are there, talk to the person running the dojo and ask to train. From here, you will be able to select from a list of characters to train against. (I would personally recommend Gonzo, just because he has a massive sword and his attacks are so telegraphed and easy to parry.) The goal here is to beat them, naturally, but also to score high. If you get over 7,000, then you will achieve the rank of Master. This actually isn’t very difficult to do; you just need to parry well, avoid damage, and keep up your attacks to finish off your opponent as quickly as possible. If you attain a score over 7,000, you will achieve the rank of Master, and the 'Striver’s License' trophy will unlock.