I need an advise about the team building

I always buy the highest price player for my team. All age from 18 - 22 year old. And all five star, at mid of season, all player can up 6 star. But I always lose in some match with weaker team (weaker 2 - 3 star).

If mod read this Thread,you can check my acc for more info.

About Formation, I choose 4-4-2 classic, 4-5-1 V style, 4-1-2-1-2 narrow diamond... and sometimes use different formation but still lose....

2 Last season, my team is strongest in League, online to play usually but end in rank 5. Can not play champion league next season.

Why?, who can give me an advise to play. Stronger for what purpose, but best player for what purpose.....