Few problems here:
- Arthur is purely offensive. She has no defensive skills.
- Offense wins fights, and that's why many fights have a minimum recommended attack power. Consider a common scenario: several waves of 3x trash mobs followed by a boss. Against trash, priority #1 is whittling down their numbers so you have 1-2 mobs hitting you instead of 3. Good offense = straight damage mitigation. Priority #2 is killing the last two before they get a painful overdrive skill off. Again, good offense mitigates damage. Against a formidable boss, your goal is to not let the boss get off an overdrive skill during rage, because those moves often do massive damage + buffs/debuffs. Ideally, that means being able to get the boss from 100% rage to stun in 1 round. Again, that requires strong offense.
- Defense without offense is risky. RNG is always your enemy because you can't anticipate it. 3 mobs alive means 3 chances for a triple attack, potentially on the same target. A triple attack from a powerful boss is almost always dangerous. Debuffs miss. The longer the fight drags on the greater the chance something will go wrong. Mordred is a good choice for sure, and Andromeda has Resurrection to recover from mishaps. But that just lowers the offensive bar a bit. Without sufficient offense you'll still lose, especially since healing is capped and damage is not.