Get rid of Arthur. At 24k total atk you dont have what is needed to burst mobs and boss fast enough, get Cassiopeia or andromeda. One of those two and nike will allow you to have a permenant increase in your defense by cycling through their abilities. Also Cassiopeia can remove enemy buffs.

I suppose roland and Joan could be useful too if you dont have the healer souls, however i would suggest everyone to invest in them since ultimate and ragnarok shouldn't be doable unless you have a super powerful team.