OK, have to admit i was wrong in my assumption that having a team of 11 (skill 99) players on your squad could actually help with this achievement.
When you have 11 of those skillful players, you will win every game that you sim. I suggest creating 11 of these players, don't forget to put them on "free agent" instead of a team! That way in every manager mode that you will start you will get the option of taking those created players with you... Follow my nine steps!
First step, creating 11 players with a skill average of 99.
(you may want to put the aggression bar to 50 only... this will save you a lot af bookings every season... ;-))
Second step, put all your created players on "free agent".
(you have to do this every time you create a player, if you forget to change to "free agent" the new player will be assigned to a team)
Third step, create a new manager mode.
(SPAIN, FRANCE, GERMANY, ITALY or ENGLAND)
Fourth step, get all of your created players and your Pro on the team that you chose. (doesn't matter wich team, every team will become a 5 star team with those players... :-))
Fifth step, sim your way through the championship.
(you CAN lose a match but only with a ratio of 1/100, so don't worry about this)
Sixth step, finish the championship and start again with the same team... BUT when you have to play your first "friendly" match of that second season QUIT! That way the game has saved your championship!
Seventh step, create a new manager mode in a different country.
(SPAIN, FRANCE, GERMANY, ITALY or ENGLAND)
Eight step, get all of your created players and your Pro on the team that you chose. (doesn't matter wich team, every team will become a 5 star team with those players... :-))
Last step, SAVE OVER YOUR LAST CHAMPIONSHIP...!!!
This way you can start a new manager for each league every time.
It was reported (360achievements) that you had to get all 5 league championships in one manager mode session, this is proven WRONG!