The first thing to do is report them in-game via the report button after a match. But if you want to go further, you'll have to follow the "right path" to report them. This means: Please record a brief in-game video of the battle, that can show us what the user does in the game to have proof and evidence of the infraction. If you don't know how to do it, check this:

Android: https://support.google.com/android/answer/9075928?hl=en
iOS: https://support.apple.com/en-us/HT207935.

Please DO NOT send us a screenshot of the results of the battle, we can't identify anything from that. Please remember that you have to always report them in-game, so the related team may receive the report ASAP. If this is not possible, you can do it via in-game support and we will address that as fast as possible. Your ticket will be addressed within 48 hours max from Monday to Friday. We love our community, we love our gamers, we want the best for all of you!