When Vergil's health is low, he will transform to demon form. This time, if he performs helm breaker (fall from sky and hit you) don't try to hit him. After devil trigger stopped, he will be calm. In this time, hit him hard. He also can transform to D.T. And perform too many judgement cuts (generate a power boost from Yamato). If he throws his sword to you, fire the sword with Ebony & Ivory.
How to defeat Vergil in mission 13 and mission 20.
Added 12 Apr 2013, ID #17824
In mission 13, Vergil attacks with Beowulf. So don't try to attack him when he is active. After a bunch of attacks, he will be calm for a second or two. During this moment, attack him. When he is in devil trigger, you also transform in devil trigger and repeatedly hit him though you may be unable sometimes. Otherwise his devil trigger will continue for a huge period. In mission 20, equip Rebellion and Beowulf, Ebony & Ivory and Kalina Ann. When he becomes calm after attacks hit him with Beowulf and perform many rising dragons. When he is in D.T, you also transform to D.T and hit him with Rebellion. In extra time hit him with ebony & Ivory or Kalina Ann. Some hints are remainig for level 20. I will submit them soon.