This game has 305 possible stars, and you only need 75. The best way to go for this achievement is to just progress through the game normally - preferrably in co-op as keeping your multiplier alive is easier there - and then go back to early levels and get 5 stars on them. The first time you do level 1, it's basically impossible to get 5 stars. If you come back at level 30 with good gear, it's basically impossible not to.
You can easily obtain all 75 on normal, as that set of levels alone has about 150 available stars.