Levelling up from 70-80 will take quite some time. Do remember that, to finish plot instance 8, you need to reach level 80.
So as of now, survival shop is pretty good place. If you do want to get him to 5 star, I'd say get him to 4 star and then buy fragments till you have 60-70, then do sweep elite instance everyday. I'd guess this way, when you reach 80 you'd have 120 frags ready.