Ive been mucking around with this ..
Buy at level 4 train to five star and then sell to level 6 then train...
Its pretty simple btw...the only difficulty is having teams similar in levels on the same server
Edit: I slipped up in the training at level 6 and used Skill drill and it % upped the heading ability which I did not mean to do .
One thing I will say if you are really going for a team of these guys you need plenty green packs and a lot of free time.