Lxzy is correct!
Stage 15 - 17 - 20 need to be Auto Friendly.
To Claim Arbiter from the missions, You have to be able to auto 10 times on those specific stages.
Stage 16 - 18 - 19 are 1 time dungeon fights.
I recommend manualling the fight to ensure you win.
All you have to do is beat them 1 time.
Than you will never go back to them ever again.
If I was you, I would try what Lxzy is saying.
You will never fight Stage 19 again.
It is just a road block to your true destination.
If you can just get lucky in 1 fight, It would be dream come true.
Yeah - Stage 20 is Spirit.
All your heroes are magic - You will have way easier time vs. Stage 20.
Stage 20 is the most important stage for farming good gear + final test in Arbiter mission.
Do you have the hero Renegade by chance?