And what buff exactly did it get? Ive been eating this team for few months, water main's changes didnt affect anything. This team isnt meta anymore for a long time though, basically ever since jinchuuriki 1&2 release. Considering current event ninjas, its even worse.

Are you assuming they buffed her since the mei meta seems unchanged to me. Have you been fighting the same people before and after 3.0? If you are comparing players via arena, you do know that people won't stop getting more power and initiative. You could make a case if you were fighting someone having a lower power and initiative but it would mean you might not have a good counter to that team in the first place.

All i know is,3.0 i was 60k,fighting 60k Mei team,and win like 85%,now in 4.0,im 70k,fighting Mei team 60k,and dont even kill 1 ninja.

Maybe there isnt buff,but something not the same with that team

yes was changed, was, infact, nerfed since tenten gnw passive now lasts 3 rounds, while in 3.0 lasted 5.... it's the only modification that team got.

Provide what team you normally used in 3.0 and in 4.0 would help out if there was something wrong. It could also be the opponent has more initiative than you so mei's barrier would be able to stay, meaning it buffs tenten and mei.

I use water main,sailor Sakura,sage Naruto,Roshi.In 3.0 i used to take both Tenten and Mei mystery,at same power,survived and win,now after both mystery my team is half dead,with 10k power advantage on my side

Wait a moment. What does it mean that you endured both mei and tenten mystery?

There's no way your roshi is able to endure Tenten mystery landed at the end of round 1 + Mabui combo + Mei mystery. It's sure that Roshi dies with no shields unless you own a very wide difference in power (and for wide difference i mean 20%+ difference), even if you own initiative and you are able to break the barrier before mabui targets you.

I'm not sure about the buff thing after 4.0.
Have you tried modifying your talents?
Were you using poison tai? If so, try experimentation so you can ignore Tenten's round 1 dmg?
You have Naruto and Roshi, they can handle the dps part.
Haven't tried this myself as I don't have Naruto yet, but it could be the better way for you to deal with those Mei teams.

A new lineup has emerged btw, looks like a combination of water blitz and water cancer team. The Roshi/Fuguki/Sailor teams.