Atk

At that point? I guess around 54 atk is the average for a +10 5*.

Higher than 55

At 5* +10, I’d say it greatly depends. Since armored units and Lukas can hit 50-60 defense naturally, higher attack is better. 55 is a good starting number, but I’d say in a competitive range you’d want the ability to be able to have at least 60 attack or a high damaging special if facing max merged armor/dragon tanks in the arena.