Is there an easier way to get stars ?

No unfortunately not however you can forefil the second and third criteria on any difficulty if that helps

as he said, and you dont need all stars. I read that (and theres 99.99999% this is true) that you can avoid 30 stars to get everything, but it doesnt give you anything special to have everything in star shop, so buy the stuff you need, if you dont like the star system. I didnt like it either but i got everything in there, and i avoided a lot of them (i felt) and still have a few over limit.

There's only one way, by playing all of the missions on hard. Because, in all honesty, getting that default star (every mission always has a star for playing it on hard) is the easiest star to attain. There's 104 missions (I think) which will, obviously, get you 104 stars if you play it on hard. There's 3+saga finishing trophies/achievements for you and your stars. There are easy missions in the first two sagas (Saiyan+Frieza), easy meaning not the beat it on hard mission. And just play everything on easy if you want to try to unlock the two other stars. I'm still finishing up (just started playing the game) so best of luck and wish me luck to :D!!!