One of the reasons why Vincent berserker is abused in the arena is his absurdly high ATK that is further boosted by his skill and aura. Sure, some might argue he compensates his sheer strength with his relative fragility. A warlord, for example, is only short by 20-30% in ATK but has more utility.
However, not many might have noticed is how DEF reduces damage dealt by your opponents. As of now, the formula for DMG-reduced-by-DEF is such that 1 point of DEF almost completely negates 1 point of ATK through flat damage reduction. This means that having a bit of both ATK and DEF is significantly worse than exceling in ATK or DEF entirely. While CRIT has helped both Shizu and Kane by doubling their base DMG when they proc crit, Freya, with her high-but-not-strong ATK, suffers immensely as she neither hits as hard as Vincent berserker nor crits reliably like Kane/Shizu.
E.g. Berserker vs Warlord against enemy with 5k DEF
Berserker with 12.5k ATK = 12.5k - 5k = 7.5k DMG
Warlord with 8k ATK = 8k - 5k = 3k DMG
7.5k ÷ 3k = 250%
Also, this would also help Lucille do a bit more damage in the arena during turns when her skill is still on cooldown.
TLDR: Rework DEF/Damage-reduction formula such that it mitigates a percentage of ATK at a decreasing rate as DEF increases, instead of flat damage reduction it (partially) relies on as of now.
Note: Attached below is a graph of how I suggest DEF should mitatigate damage done by ATK. Figures to be adjusted.