As for how you did so little damage, I'm not sure (you might just have been unlucky) but for their minato they probably did so much damage because of kakashi's buff, since you'll have 30% less def/res.

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It's arena, crazy stuff happens there that never happens in real PVP.

Go auto in arena, anything else is a waste of time.

Fight for real when you stats actually matter.

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Arena MOSTLY, strips you of your equipment/runes/Lee Paths/summon bonus that pump you up. But things that have been added since early in the game (Treasure tools, Myoboku, Assist links, Five Nature Chakra, Ninja Assist, Secret Scroll) will all be added to your team's stats. So, older and $$$ players will still have an advantage. Those players that have maxed out those new things are going to do just what you saw, take ~50% damage from you, but completely wipe you out in 1 attack. The star level of the ninja is not going to impact the damage done hardly at all. <100k power star level is a big deal, after that....not so much.

I don't even like OP, but you have to admit, it's unlikely that their opponent had higher stats on the things you mentioned that count towards arena. Someoneone running 3* Minato and Kushina is not going to have stronger Treasured Tools, Assists, Five Natures, etc., than someone who has been playing for years and has put money into the game. Surely there must be another explanation. The only thing I can think of is something to do with their Edo Madara cancelling buffs, but it really shouldn't matter because his barrier would be overriden by Kimimaro's and Kakashi's buff only happens in round 2.

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!