No need to train after 7*. Wait until the new season, then he loses one star. ( If you are promoted, if not, then he will remain the same quality. )
There is a tactics to make your ST, AML and AMR 8*, and let the rest of the team live on leftovers, like, 6*. I'm not saying it's a good tactics (I haven't tried it... yet), but tomorrow I'm playing against such a guy. Again. The previous one beat the sh1t out of me.