Hello,
Speaking from experience, I've seen the edo Madara/Kimimaro/Minato team before, and it can be pretty destructive in arena. I'm not surprised that it can destroy a summer Kakashi team in arena on round 2. Kakashi's defense reduction passive hits him very hard. I'm not sure you're all looking at the math properly when considering how it works.

Let's assume that all ninjas involved have stats of 1000 in everything. Edo Madara assures that all buffs are wiped, so everyone goes into the match with the same stats.
With Kimimaro's barrier up, mystery damage is increased by 60%, so stats are effectively 1900 for the purpose of mystery damage with Minato's +30% increased mystery damage.
Your team's defensive stats are effectively 700 for the purposes of taking damage from mysteries. He has 2.7 times your defense stat at this point. It's no wonder you're taking so much damage. If he was smart and running Pose of Duel and Deep Forest Wall, that number increases to 3.14 times your defense stat.
Conversely, your opponent, who knew he would be wiping buffs, is almost certainly running Defense Experience - Ultimate for a 40% boost to defense and resistance, meaning his defense stats are effectively 1400 compared to your 2500. You have only 1.78 times his defense stat. This is why you did so much less damage.

Ultimately, summer Kakashi has the potential to do a ton of damage, but without position 1 scaling, he can be pretty squishy, and it seems possible that your opponent just had a team that countered yours pretty well. It's also worth mentioning that you took damage in round 1, which would help account for your team being wiped out in round 2.

What brensonlee said is not incorrect. Many units function very differently in arena than in normal pvp. The difference in scaling and the lack of position 1 stacking causes huge changes to the game. Teams that work in regular PVP can be completely invalidated in arena, and teams that are unbeatable in arena are useless in regular PVP, and vice versus, just because of how scaling and control works in this game mode. It's something all players have to adapt to.

I hope this helps!