Naruto is learning the elements of Wind Style Jutsu, and Kakashi lectures him on the difficulties of developing a new jutsu.
And here I need a ton of concentration
just for the Rasengan.
Necesito concentrarme muchísimo 
sólo para el Rasengan.
Necesito concentrarme muchísimo
 solo para el Rasengan.
Soltanto per il Rasengan
mi ci vuole un sacco di concentrazione.