This is going to take time to build your team regardless of what way you do it. In order to have a 5 star team your Team Potential has to be around 87.5. There are two ways to do it.
1. Build a 5 star team by getting players that are young with high potential. You want to build the cheapest but best team possible. The salary cap is 25'000 which makes it very hard; hence the reason you want to "grow" your young players. To do this you are going to need good players, and then a alot of training consumable cards, and I mean ALOT. This may not be the best method but is one way to do it. Enter EAUHL and match up with a 5 star team. ex OFF-90 DEF-89 GOA-89. Beat them and the achievement is yours.
2. Easy to get a 5 star team, just buy card packs and trade for players with high ratings. Even if they only have 10 games left you only need to win 1, so trading for people's leftover cards isn't a bad idea. There are 2 online tournaments where the salary cap isn't a restriction. These are "United Nationals Trophy" and the "Minor League Championship". The only catch is in order to play these tournaments you have to unlock them by winning x amount of tournaments. The easier being United Nationals which only requires you to have 1 tournament win, which can be any tournament of your choice, online or offline.Once you unlock United Nationals, Enter the tournament and play a team that looks like they would be a 5 star. ex OFF-90 DEF-89 GOA-89. Beat them and the achievement is yours.
I built my team by legit and used the second method. I actually made a kid quit before the game was done and the achievement unlocked. Good Luck!