Hm. If I were in your place, what I think I’d do is this:
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Check to see if they’re also getting seemingly-impossible wins in war attacks, and make note of how many questionable wins they’re getting over a period of a few days in surge, as well as noting how many (if any) they’ve gotten over the past month of war.
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Ask them a question about one (or some) of their wins, being sure to phrase it as genuine curiosity about hopefully learning some tips to improve your own play. This will be a lot easier if you see a war fight win that you can ask about, but even if they just do it during surge, you can say (e.g., but use a more recent fight) “I always struggle against Shego and Zeus, but I saw that you beat a team with those two and some other troublesome heroes in surge without even using Mary(Lo). Do you have any tips I could use?”
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If you still feel something unethical is going on, contact support and give them the information you’ve collected. Don’t expect a fast resolution, or even any information about what they do with your ticket, but they could well take action with them ranging from warnings to bans.
A couple of years ago, one member of a guild we frequently faced in wars kept getting these really improbable wins against us, using older, under-leveled heroes with no synergies to beat teams with high-level meta heroes. I ended up asking support to look at things, if possible, to make sure they were legit wins, and while that player never left the game, the last several times we faced that guild either he didn’t attack or he attacked much weaker opponents.