Do you honestly think Grey isn't any good?
No one is saying emotions are the main aspect of winning a fight. It wasn't in the chapter we're talking about, it wasn't in the games. It is one of many factors and it can be important enough to make a weaker team/character win. And "intensive training" happens all the time in soccer, man. In the brazilian championship there is not even time to change training methods (1-2 games a week). What you don't seem to notice is that no one (not even Mavis) is saying "emotions trump everything else". It's just that emotions can be that extra something that might make you win. You know, like it happens in every shounen manga about fighting, ever.
Drolyt, it looks you just don't like emotions affecting the outcome of events, but they do so in real life (that's why every professional sports team has psychologists, that's also why Adriano The Emperor left Europe - he was depressed and had lost the joy in playing soccer) and that happening is a staple of the shounen genre, one of it's main tropes. If you don't like that kind of thing, well, then you're reading the wrong genre, because that is going to keep happening.