That what i noticed is, in arena main idicator is you current player level. Doesnt matter on which arena level i'm always get roll of opponets -3 +3 of my current player level. My coclusions are base on my fights (need 400 silver so focuing on arena for cuple of week now, have 75 silver so far :D )
I aslo use speed team with 70k, so now i have chance to crack open and kill some of other speed teams.
But have opponents with 90+k, for which i dont have enoguh nuke power to kill.
Other thing what i noticed that opnents team composition depends on your team composition:
When i didnt have HK on lead - had 90% of speed lead teams each roll, then i changed to HK as lead now im facing only 30% speed teams, others are mostly 90k with high def/hp, which i cannot nuke :/