Ple-Sen
Even with 10 attacks, I can't stand a chance against you. During their turn, the opponent can't deal more than one point of damage, which you will recover at the beginning of the next turn. The only hope here is:
1) Negative effects. It's necessary to inflict a wound on you + some other negative effect and hope that the katana won't remove the wound.
2) Overwhelm with a crowd, which is impossible on a solo arena.
3) Hope for a chance of a "critical hit" or some similar effect.
4) Destroying your katana.
5) Mega-high Yin and a lot of luck, so that you can go at the beginning of the next turn and before the healing trigger, but I don't remember any such monsters on the arena yet.
I think it's not worth saying that the chances of anything from the above happening on the arena are not that great.