Unit Captain must be fought twice and in both battles only The Mask and Glaive are your allies. The first battles occurs with only Glaive. Fight him for a while and he will use Achilles Strike and reduce Glaive's Hp to under 10. Then The Mask will appear and fully heal him before the battle continues. During the second battle, a Lancer and a Martial Artist will support him.
Unit Captain isn't specifically stronger than the other foes on the area. The only disadvantage you have is that you can control only 2 characters during both fights. It's not really a big problem. All you have to do is healing your team in time and eliminating all the supporters first.