You can improve white skills until 140%-160% with rests and smart choices of trainings.
After that with 20-30 friendlies per day you can get other 100% skills point on whites. In a week or two you have a mutant.
Be sure that there are people that does 20-30 friendlies.

After that season by season you need few friendlies to keep it as mutant.

I don't do it as I prefer to have a life instead of making tons of friendlies......