This one can be frustrating, since most enemies, groups or otherwise, die far faster than you can reach Phantom Rush.
Note: If I'm not mistaken, it actually requires you to fill your Phantom Rush gauge 50 times (not merely do 50 Phantom Rush attacks; you can do multiple Phantom Rush attacks during a single charge of Phantom Rush if you keep your combo going). If I am correct, then you need to let your gauge empty before starting up another Phantom Rush.
Boring, but fast method (available from the start of the game): Go to the base of the Asura and attack its feet. You can attack, and even Phantom Rush, but will deal no damage so you can just do this as long as you want. It also can't attack you there, so you are totally safe.
Slow Method: Play the game and fight the Guardians and Titans a lot, making sure to do as little damage as possible to charge up the gauge.