Basic advice I give anyone:
1)focus on hitting flawless hits as much as possible, even if you have to skip some that wouldn't have been flawless
2)unless you're in levels 4 and 5 (and to some extent level 3), in which case you need to get the combos even if they are not flawless hits. As a matter of fact, many combos are available only if you don't hit flawless on all of the fireworks in these later levels.
3)if you can get flawless hits while doing the combos on any level, then that's even better!