I applied 5 steps to kill nemesis.
1/ clear all map except nemesis. Move all people around him.
2/ control claude to take a bow in the center and force nemesis out of his position.
3/ rally defend for raphael, move others out of nemesis, but force him to attack only raphael.
4/ raphael will survive after nemesis attack because his def is very high. Heal him and use raphael's combat art to heal himself. Goal is to destroy his sword
5/ after destroy nemesis's sword, i play with him as long as I like. Without sword he just make 25 30 damage per turn.